Laser interferometer with picometer precision is a key technology in the space gravitational wave detection. Many interferometry strategies have been put forward for the multiple purposes in the past 10 yr. We propose a new interferometry method, called constant amplitude modulation (CAM) heterodyne interferometry. Differently, the CAM provides an optical pilot tone (OPT) for the noise correction theme. Compared with the analog pilot tone, the OPT can record and correct more noises, such as the analog to digital converter sampling jitter, the photodetector noise and the analog front-end noise. From the discussion, the modulated depth ϕmodulate=1.375 rad and the power ratio of the beam split n=0.432 are the best choice for the CAM-modulated parameter. Moreover, a simulated case has been implemented for the verification of the CAM strategy. Therefore, the CAM gives us another excellent choice in the optical design of the interferometer.