Most textbooks use two objects to collide completely in-elastically for observing the speed

change from different frames of refer- ence，and obtain the mass-speed relationship. This paper

uses the Lorentz transformation of space-time in the spe- cial theory of relativity to prove the

invariance of the force in the direction of motion in the two inertial reference

frames. Then，Newton's second law is directly to derive the relationship between mass and

speed. This derivation method is relatively simple. which is very conducive to the

understanding of junior college students.