The tension causes pulley eccentric in belt drive. Through the analysis of the shaft deformation caused by the pulley eccentric, this paper resulted in the calculation method and related formulas for belt drive rotary motion error under different tension. And based on the PDJ-4 belt drive experiment facility giving examples of analysis, we draw the conclusion that with the increase of tension, rotary motion error increases at the same time. Thus it will have a greater rotary motion error when the tension is too large. This paper proposed the scheme and measures to improve that error eventually.