A locomotive constant-speed control method and a locomotive control system are provided. The method comprises a step 1 of acquiring a current locomotive speed and calculating a difference value between the current locomotive speed and a preset locomotive speed to obtain a first speed difference value; a step 2 of acquiring a current locomotive load and determining a current adjustment coefficientaccording to the current locomotive load and a preset nonlinear adjustment curve, wherein the preset nonlinear adjustment curve is a function relationship between the locomotive load and the adjustment coefficient; a step 3 of determining a second speed difference value according to the first speed difference value and the current adjustment coefficient, and outputting a corresponding set force according to the second speed difference value by using a preset speed control model in order that the speed of the locomotive remains constant. The method enables a passenger locomotive to adapt to different locomotive loads under a constant-speed condition, and improves the accuracy and stability of the locomotive constant0speed control.