To study the unlined elliptical tunnel stability, strength reduction method was introduced into the finite element upper bound method. The relations between safety factor and influence factors such as shear strength parameters, buried depth and span were studied with self-made program. Meanwhile, the failure modes were also explored for limit states derived from strength reduction method. The results show that strength reduction method can be easily embedded into finite element upper bound method. The relation curves have shown a positive correlation between rock safety factors and shear strength parameters, while rock safety factors have negative correlations with buried depth and span. The influence of the tunnel span on the safety factor is obvious, and the reduction in rock safety factors has been found to vary approximately in a range of 27%~46% when the ratio of tunnel span and height (B/D) increases from 0.5 to 2.0. The failure modes with distinct characteristics reveal that the main slip surface starts from the bottom or the side wall of the tunnels and then extends up to the ground surface. The slip lines are mainly focused on the top of the tunnel arch and side wall. Meanwhile, horizontal extents of the damage area are less than 1.5 times of tunnel height.