Taking the construction of the Dalishu Expressway tunnel entrance as the engineering background, an investigation has been made of the instability and reinforcement technology of the entrance working face slope with ancient landslide factors taken into consideration. By using borehole inclination data (BCD) and Midas GTS reduced strength finite element analysis model, the most dangerous sliding surface is speculated separately, with the results showing that the positions of the two sliding surfaces are found to be similar. By using unbalanced thrust transfer method, safety factor can be calculated based on BCD inference of the sliding surface. Based on the sliding surface of BCD and the finite element method, the sliding surface is approximately assumed to be a logarithmic spiral, considering terrain factors, with a multi-step slope upper limit analysis model established to study the impact of different reinforcement measures on slope stability. The results indicate that there is a similarity berween the slope safety factors calculated by the three methods.