In the design of the Offner stretcher, it is very important to investigate the mirror curvature radius errors, which has a direct impact on the quality of the output pulse. So it is necessary to analyse the effect on the output pulse contrast due to the mirror curvature radius manufacture errors in the ffner stretcher. A ray-tracing model for ffner stretcher with these errors is established. Based on the model, effects on output pulse contrast through the stretcher is analyzed.When the mirror curvature radius error is between 0.2%～0.5%, it can fully meet the requirements for the SHENGUANG Ⅱ petawatt laser facility. Also, when there is error for the mirror curvature radius, keeping the distance between concave mirror and convex mirror half of curvature radius of the concave mirror is better than keeping the two curve mirrors in the same center.