As solar light was mainly absorbed by absorb layer in thin film solar cells, the quality of absorb layer has a great effect on the performance of solar cells. Based on the diffusion drift model of carrier in copper indium gallium selenide (CIGS) solar cells, and then the key parameters are analyzed. Combining with the experimental results of others, this paper discusses the absorption layer’s thickness influence on the open circuit voltage, short circuit current, filling factor and conversion efficiency of the copper indium gallium selenium based solar cells. The results show that the absorption layer’s thickness always affects the performance parameters of the battery. Open circuit voltage increased with increasing thickness and reached saturation above 2μm. Short circuit current increased first and then decreased, then reaches its maximum value at about 2μm thickness. Conversion efficiency is similar to the short circuit current properties. While the fill factor decreased with the increase of thickness above 1^m. This can provide theoretical support for the design of better device structures.