Nonnull test is often adopted in optical metrology for aspheric testing. But due to its violation of null condition, the obtained surface figure would be different from that of the real, which is called retrace error accordingly. Retrace error of nonnull aspheric testing is analyzed in detail with conclusions that retrace error has much to do with the aperture, relative aperture and surface shape error of the apsheric under test. Correcting methods are proposed according to the manner of the retrace errors. Both of computer simulation and experimental results show that the proposed methods can correct the retrace error effectively. The analysis and proposed correction methods bring much to the application of nonnull aspheric testing.