In this study, a tunnel line slope angle is introduced to optimize the traditional silo model, and an instability model of rectangular pipe jacking excavation surface is established considering the effect of pressure inside the silo during uphill tunneling. Both these approaches aim to investigate the stability of excavation surface of rectangular pipe jacking during uphill tunneling. Based on the limit equilibrium theory, the pressure expression of bulkhead soil chamber is derived, and the influence of soil shear strength parameters and line slope angle on the stability of excavation face is discussed. The parameter analysis shows that the pressure of soil chamber decreases nonlinearly with an increase in effective internal friction angle of soil and increases linearly with an increase in slope angle of the line. Comparison of the proposed instability model with the existing theoretical model and numerical simulation shows that the proposed model is superior in predicting the failure slip surface at the bottom of the tunnel. The instability model is verified based on a rectangular pipe jacking project in Hangzhou. The comparison results show that the instability model has good engineering adaptability.