Taking Xi'an region for example, four influence factors on real raindrops falling in the Earth atmosphere were studied theoretically. Three groups equations of raindrops established with the Newton's dynamic equation in standard coordinate system were numerically solved through the MATLAB. For the small, middle and big raindrops, the terminal speeds reach the ranges within (2.1-5.6m/s), (6.0-8.5m/s) and (8.7-10.5m/s), and corresponding to the falling height scales of (1.0-8.0m), (8.2-17.0m) and (22.0-33.0m), as well as the eastward migration ranges of (170-590m), (90-150m) and (55-75m), and corresponding to the southward migration scales of (3.2-16.0m), (1.0-2.1m) and (1.7-0.9m), respectively. As per the t-test and Poisson coefficient, the above three models are favorable. The theoretical research will promote water conservation in the Loess Plateau.