To discriminate and measure the whole stability of unlined shallow earth tunnel in theory, considering the effect of oblique slip surface beside tunnel body, a theoretical solution of ultimate surcharge is deduced by adopting the limit equilibrium method during the occurrence of the tunnel overall instability, and then the solution is examined using the typical loading tests. It is found that the solution of ultimate surcharge formulation is close to the test results. On the basis, the ratio of sliding resistance to sliding force is defined as the whole stability safety factor, and its influencing aspects are analyzed. It is shown that the stability safety factor increases along with the increment of the cohesive force and the internal friction angle, and the effect of cohesive force on the safety factor is significant, but the effect of internal friction angle is less potent. The stability safety factor decreases with the increment of the span, the depth-span ratio, and the surcharge of earth tunnel, but the surcharge has less effect on the stability safety factor when it is larger. With the increment of the height-span ratio, the stability safety factor decreases as the cohesive force is low, but it increases firstly and then decreases as the cohesive force is high. Finally, the whole stability safety factor is verified by typical loading experiments, demonstrating that the calculation error is within the engineering allowance.