Terrestrial Laser Scanner（ TLS） technology can quickly acquire three-dimensional information of targets with high precision. Given that TLS is a new data collection technique

it has been gradually applied to characterize the structural attributes of forest canopy. However

the inversion accuracy of Leaf Area Index（ LAI） is highly dependent on the intrinsic configuration of the sensor

such as beam size and echo detection energy. In this paper

a computer simulation model was proposed to simulate point clouds from TLS and to analyze quantitatively the influence of beam characteristic on LAI inverted from TLS data.A realistic tree was generated with Onyx TREE BROADLEAF software. Moreover

a computer model was proposed to simulate the interactions of lasers with a single tree and to acquire the point clouds from a TLS Riegl VZ-1000 based on the ray tracing algorithm. This model consisted of the ray intersection with triangular patches of photorealistic trees

the coordinate system conversion

and the acceleration of the algorithm. The beam size at exit

beam divergence

and echo detection algorithm were considered in the computer simulation method. One laser beam was divided into multiple bins

and each bin was treated as a separate pulse with its location

propagation direction

and an initial energy changing into a Gaussian shape. We inverted the crown-level Leaf Area Index（ LAI） by using gap fraction analysis with the simulated point clouds

and the influence of beam characteristics（ such as beam diameter and minimum echo detection intensity） on the LAI inversion was analyzed. Finally

we conducted the validation with the measured points of a birch tree located in Root River. We analyzed the influences of beam characteristics

such as beam size

beam divergence

and echo detection energy

on LAI inversion. The inversion results indicate that beam size and detection limit greatly influence LAI inversion. The points are increased with the decrease of the corresponding gap fraction because several points can be returned from one beam when the beam width and divergence were considered

particularly when significant differences are achieved at the edge of leaves. A larger beam size means that components in the edge portion are intercepted more easily. Thus

the deviation of LAI inversion would be greater. When the detection intensity threshold was small

echo information could be returned even if only part of the spot edge was intercepted. Thus

gap fraction is undervalued. However

when the energy threshold setting was large

the returned energy may be below the threshold value and cannot be recorded

thereby resulting in overestimation of the gap fraction and underestimation of LAI. Therefore

the points caused by beam size and echo detection must be filtered

and suitable points must be chosen before inverting LAI with the gap fraction model.The simulation model based on the ray tracing algorithm was presented to explore the laser beam interceptions with an individual tree generated by using Onyx Tree software. The LAI was retrieved via gap fraction analysis with zenith slicing method. The beam characteristics

such as beam size

echo detection energy

and beam divergence

were considered. The simulation model enables efficient and cost-effective research that can avoid environmental and instrumental error. This model contributes to an improved understanding of the intersections of laser beams with the tree crown well

and the LAI inversion of an individual tree is facilitated.