The invention discloses a constant-tension control method for a diamond line cutting machine. According to the constant-tension control method, the swing angle error e serves as the control basis, and the control target is to enable the swing angle error e to be 0; the feed-forward datum angular speed omegat is obtained through feed-forward control; the angular speed initial fine adjustment amount domega is obtained by utilizing the PID control algorithm through feedback control; the angular speed soft-limiting processing fine adjustment amount domegaL is obtained through soft-limiting processing; and finally, the feed-forward datum angular speed omegat calculated in feed-forward control is superposed with the angular speed soft-limiting processing fine adjustment amount domegaL, and thus the target angular speed omegay of a line roller is obtained. According to the control method, it can be ensured that line routing of the diamond line is steady and line routing tensionis constant, and tension fluctuation is avoided.