To analyze the deformation of the tunnel in weak loess, the surface settlement, crown settlement and horizontal convergence are in-situ monitored in the Dayoushan loess tunnel by using precision level and convergence gauges. The results show that the magnitude of crown settlement is much larger than that of the horizontal convergence. The deformation sustains for a longer time and the steady maximum crown settlement is 950.6 mm. The deformation of surrounding rock is larger when the buried depth of the tunnel is at the critical depth. The deformation rate of surrounding rock is larger at the stage of secondary lining construction. The deformation rate, which acts as the stability criterion for the surrounding rock-supporting system in the weak loess tunnel, should be raised moderately. The deformation of the surrounding rock follows an exponential law; and the exponential function can be used to predict the final deformation of surrounding rock. The deformation of weak loess tunnel can be divided into three stages: rapid deformation stage, sustained growth stage and slow growth stage; and finally, it tends to steady. The tunnel excavation significantly influences the surface deformation; and the maximum settlement of surface occurs along the central axis of tunnel; and the settlement gradually decreases with the increase of the distance from central axis transverse. The deformation allowance of weak loess tunnel should not be unified at different locations. For the weak loess tunnel in Xining area, the reserved deformation of grade Ⅴsurrounding rock is suggested to be 700-800 mm for the crown, and 300-350 mm for the side wall. The crown and side wall of tunnel should be connected by smooth curve.