The higher the temperature of a blackbody, and the greater the amount of radiation of the terahertz band, the higher the test signal-to-noise ratio, but at the same time the increase of the amount of infrared radiation is far greater than that of terahertz radiation. The infrared radiation is difficult to filter and the blackbody terahertz radiation test accuracy is reduced. To improve the test accuracy, a suitable temperature range is chosen for the blackbody and the test method is designed for its terahertz radiation. Firstly, the distribution of spectral radiant emittance in the terahertz and infrared wave bands is calculated by Planck′s law for the blackbody at temperatures between 3~600 K. The radiation difference of blackbodies in this two wave bands is analyzed. Then the relative changes of radiation at different terahertz narrow band wavelengths for blackbody at the same temperature are analyzed. The phase-locking test method for blackbody in terahertz band is put forward, with the suitable blackbody temperature range of 223~273 K. The experimental results show that the terahertz test relative deviation is 0.1%～2.0%. The terahertz test curve is consistent with the related theoretical curve for blackbody in 100~3000 μm at this temperature range.