Abstract: According to laser photo-thermal method, the measurement system of high temperature superconducting (HTS) tapes thermal diffusivity was built in low temperature and vacuum condition. Through the research on the relationship among thermal diffusivity, modulation frequency of heating laser and phase difference of heat wave when passing through specimen, novel idea to deal with experimental data from the thermal diffusivity testing by using laser photo-thermal method was put forward, and the systematic error influence was eliminated. To verify the reliability of the measurement system, the copper was used as reference material and the thermal diffusivity of copper was measured. On that basis, the thermal diffusivity of YBCO HTS tape was measured by using laser photo-thermal method, and the thermal diffusivity of YBCO HTS tape has been got when the temperature scope was from 40 K to 120 K. The thermal diffusivity decreases with the temperature increasing. Especially, decreasing sharply in the temperature range of 40-60 K.