Abstract: In order to improve the internal force of lining structure and reduce the lining thickness of high-filled trench type open-cut tunnel in loess, it studies the variation law of internal forces and lining thickness of structure as well as the effect law of slope angle on internal force of open-cut tunnel and lining thickness before and after load reduction from the perspectives of mechanical behaviors and load reduction mechanism of EPS board by numerical simulation method. The result shows that the internal force and lining thickness of high-filled open-cut structure in loess increase with an increase of filling height; reduction effects of internal force and lining thickness of open-cut tunnel under load reduction are related to EPS board thickness and stress-strain curve, the load reduction effect is the best when EPS board is at the end of plastic stage, and an optimized design for load reduction of EPS board under this density was presented; the greater the slope angle, the smaller the structural internal force of open-cut tunnel before and after load reduction; the lining thickness of open-cut tunnel tends to decrease except the vault; thickness reduction effect of the lining at tunnel roof is most obvious after load reduction while the thickness reduction of other sections decreases in an accelerate way. It is suggested that designs for different EPS board density and thickness should be optimized regarding different filling heights and slope angles of trench type high-filled open-cut tunnels.