In recent years,the monocular stereo system based on bifocal imaging is becoming an important branch of computer vision.It has a widespread application prospect.Monocular stereo system get the depth estimation of objects by two images，which are captured at two different focus.Based on the principle of the depth-estimation,we analyzed the factors that influence the range accuracy and then proposed a new ranging algorithm.In the algorithm,the object area was used instead of the image vector,which is used by traditional depth-measurement algorithm.By this way,the process of the tedious camera demarcation was avoided.The experimental results indicate that the new algorithm not only simplifies the depth-measurement process,but also enhances the accuracy of depth-measurement.