Objective To quantitatively investigate the motor dysfunction after brain stem pyramidal tract injury in rats by grasp strength (GS)-motor evoked potential (MEP). Methods Thirty healthy male SD rats were randomly divided into 1.25 m injury group, 2.25 m injury group and normal group. The classical marmarou model was used to make the traumatic axonal injury model of pyramidal tract. In injury group, the limb GS signal and MEP were detected at 1st , 3rd, 5th , 7th , 14th, 28th and 42nd day after injury, and the control test was carried out at the corresponding time after anesthesia. Results With the increase in injury degree, the MEP amplitude and maximum GS decreased significantly in injury group. The measured values of GS and MEP under two heights had obvious positive correlation. When the damage degree was relatively small, MEP was more sensitive at early stage, but MEP was not sensitive at late stage. When the damage degree was relatively large, MEP changes at both early and late stages were more sensitive. Conclusions The combined evaluation of GS-MEP can provide an data support for the quantitative evaluation of motor dysfunction after pyramidal tract injury.