Based on the physical model of a laser diode (LD) modulated utilizing photothermal effect, the relations of the photothermal intensity-modulation depth of the LD with the modulation frequency and injection current are theoretically and verify experimentally deduced. It is useful for determining appropriate modulation frequency and injection current in LD interferometer with a photothermal modulation to reduce the influence of light-intensity modulation and realize nanometer-accuracy interferometer measurement.