In precision measurement, the existed error of measuring system influences surface accuracy more. This paper provides an error analysis focusing on the research effort on aspheric lens on-machine measurement for form accuracy. And the error parameters including positioning error of coordinate origin, tilt angle and radius error of probe head are investigated. It was known that it was quite difficult to obtain the parameters by detecting instruments, so the method of error separation and correction based on measuring standard parts is presented in this paper to guide the inverse solution for higher measuring accuracy. Also, a measuring experiment with Φ210mm lens is executed. Experimental results indicate that the method based on standard parts is suitable for on-machine precision measuring of aspheric lens.