A structure of crystalline silicon thin film solar cell consisting of one-dimensional back graitngs and alumininum metal reflection was designed. Silicon,silver and aluminium were used as the back gratings′ materials respectively. The finite-difference time-domain simulations were conducted to compute crystalline silicon′s optical absorption between wavelength of 300 nm and 1100 nm. To analysis the mechanism of optical absorption enhancement,magnetic field intensity distrubutions above wavelength 600nm were plot. Normalized optical absorption density was defined to measure optical absorption of full band quantitatively. Together with short-circuit current,it was used to compare full band optical absorption of three kinds of crystalline silicon thin film solar cells. In contrast,aluminium back gratings enhance crystalline silicon′s optical absorption most efficiently.