Abstract: In order to achieve high-accuracy measurement of the long focal length, a method of measuring long focal length based on Talbot-Moireffect was studied in this paper, a long focal length test device was established, it included a collimated wavefront generator, a Talbot interferometer and a imaging system. The effect of wavefront aberration on accuracy of focal length was analyzed. Then the distance s from the sample to the grating G1, the space d between grating G1 and grating G2,and angle of two grating lines as influencing factors on accuracy of focal length were analysed. The alignment method to improve the accuracy was proposed. Pentaprism method for the detection of wavefront was introduced, to ensure that the collimated wavefront quality better than a wavelength. The linear grating encoder was used to measure s and calibrate d, to make the accuracy of the distance better than 0.1 mm. A reference reflective concave sphere was used for angle calibration, to make the accuracy of angle better than 0.001. The experimental results show that the relative error is less than 0.35% when measured focal length is 5436mm.