For p-CdTe/n-CdS solar cell, it is reported that the quantum efficiency and the collection efficiency are not only wave length dependent but also most importantly voltage dependent, since the CdTe solar cell is believed to be the diode which has non-shallow acceptors and deep levels where the roles of these levels are not clear. In this study, the quantum efficiency of CdTe solar cell with various optical biases, which is titled as "Working Quantum Efficiency (WQE)", is measured. The result is compared with industrialized amorphous silicon solar cell. Simulation models are given to explain those measurements. The result shows the measurements of WQE is one of important evaluations for CdTe solar cell as well as it can contribute to its characterization and improvement.