A CdSiP2 (CSP) crystal was grown by the modified Vertical Bridgman method. Four wafers which were cut from the crystal with the [001] direction and annealed under four different atmospheres, i. e. vacuum, cover-up with CSP polycrystalline powder, P/Cd (atomic ratio of 2:1) and Cd atmosphere. Then, Energy Disperse X-ray Spectroscopy, Fourier Transform Infrared Spectrophotometer and the Mapping System of Infrared Microscope were employed to analyze the properties of the CdSiP2 wafers before and after annealing, including the elementary composition, infrared absorption coefficients and infrared transmission uniformity. The measurement results show that all the stoichiometric deviation and infrared absorption coefficients are reduced, and infrared transmission uniformity of CSP wafers are improved to different extent after annealing. For the infrared absorption coefficients in the range of 1.29-2.00 μm, the most obvious decrease is due to annealing under cover-up with CSP polycrystalline powder. Meanwhile, the wafer treaded by this method also obtained the best result for infrared transmission uniformity in 1.92-1.98 μm with the increase of 14.06%. The wafer annealed under the Cd atmosphere has the lowest infrared absorption coefficients in 2.00-6.50 μm among the four wafers, and has the most obvious improvement for the infrared transmission uniformity in 2.70-2.78 μm with the increase of 17.43%. The main factors resulting in nonuniformity absorption in the wavebands mentioned above and corresponding effective annealing processes will analyze in this work.