Effects of quantum well number on spectral response of InGaN/GaN multiple quantum well (MQW) solar cells are investigated. It is found that the strain relaxation by forming defects occurs when the number of quantum wells (QWs) increases to 30. It results in the low luminescence intensity and external quantum efficiency (EQE) due to the poor crystal quality of MQW region and the high non-radiative recombination rate in MQW region. However, the EQE of 25-period InGaN/GaN MQW solar cell with relatively high crystal quality of MQW region does not increase at all wavelengths compared with 15-period one. This may be attributed to the lower carrier collection efficiency resulting from the lower electric field of i-MQW region.