Body size parameters are important indicators to evaluate the growth status of sheep. How to achieve the measurement with non-stress instrument is an urgent and important problem that needs to be resolved in the breeding process of sheep. This paper introduced corresponding machine vision methods to measure the parameters. Sheep body in complex environment was detected by gray-based background subtraction method and chromaticity invariance principle. By virtue of grid method, the contour envelope of sheep body was extracted. After analyzing the contour sequence with D-P algorithm and Helen-Qin Jiushao formula, the point with maximum curvature in the contour was acquired. The point was chosen as the measurement point at the hip of sheep. Based on the above information, the other three measurment points were attained using four-point method and combing the spatial resolution, the body size parameters of sheep body were acquired. And the contactless measurement was achieved. The experimental results show that, the proposed method can effectively extract sheep body in complex environment; the measurement point at hip of sheep can be stably determined and the height of sheep can be stably attained. Due to the complexity of the ambient light, there still exits some problems when determining the shoulder points.