The visible light indoor positioning system based on received signal strength(RSS)has positioning error due to the disturbance of the wall reflection and the external noise. This system applies Phong in computer graphics to the wall reflection of visible light positioning. Comparing signal strength from LEDs directly with the reflection and noise simula-tion at the receiving,it is found that signal strength of the wall reflection and direct strength are in the same order of magnitude. Thus the disturbance of the wall reflection and the external noise cannot be ignored. This article compares the simulation error caused by the disturbance to the positioning system with Cramer-Rao bound which represents the lower limit of the positioning error. It is found that maximum error is 1.8 m and the average error is 0.12 m. The disturbance phenomenon is obvious in the corner of the wall.