Abstract: A diode-pumped injection-seeded Tm,Ho:YAP laser was reported. The output single-frequency pulse energy of 2.8 mJ at pulse repetition frequency of 100 Hz with a pulse width of 289 ns was obtained. Using the injection-seeded Tm, Ho:YAP as the emission source, a laser Doppler velocity measurement experiment by means of heterodyne coherent detection was demonstrated. The detecting target was an air fan with diameter of 10 cm and the maximum linear velocity of 20.4 m/s. The Doppler frequency shift, carrying the velocity information of the air fan, was achieved from heterodyne signal between the local oscillator and the scattering light from the air fan. Under various velocity conditions, the linear velocity of the air fan was achieved from the experimental data. Compared with the actual speed of the air fan, the measurement error of experiment was less than 1 m/s.