Based on the principles of off-axis interferometry and common-path geometry, a laser diffraction phase microscopy system is designed and built up, and the spatial noise standard deviation of the system without sample is measured as 6.70 nm. Standard polystyrene spheres with diameter of 4.8 μm are used as the calibration samples, the measured phase shift error does not exceed 4% compared to the expected results. 100 Giardia lamblia cysts are measured using this system and the unwrapped phase images are reconstructed, the dry mass of each cyst is estimated on the basis of phase image. Statistical analysis results indicate that shape of Giardia lamblia cysts appear to be elliptical, the major and minor diameters of cysts range from 8μm to 15 μm and 4 μm to 7 μm respectively, and the dry mass is calculated to be in the range from 42.70 pg to 137.07 pg. The proposed diffraction phase microscopy has a fast acquisition speed and a high accuracy and can meet the requirements of the quantitative analysis of morphology and dry mass of the single protozoan parasite.