A novel palpation-based tumor detection method for robot-assisted minimally invasive surgery (RMIS) is proposed in this paper. A tactile stiffness sensor based on piezoelectric vibration is designed to detect tissue stiffness just by gentle contacting, without the risk of injuring the organ. A novel multi-tumor detection method is proposed to solve the problem that existing tumor detection methods can only detect a single tumor. In addition, the moving distance of the sensor during palpation is introduced into the sampling strategy function as a new factor, so that the total moving distance of sensor can be considered and optimized during the tumor palpation process. Simulation studies are carried out to compare the new method with existing methods in tumor detection. It shows that the proposed method can identify and locate multiple tumors successfully with the highest performance (F1 score > 0.99), and the total moving distance during palpation is reduced by 43% without any compromise in the detection accuracy.