The avalanche multiplication effect is the key mechanism of 4H-SiC avalanche photodiodes and power semiconductor devices. As the most important physical parameter, the accurate analytical expression of avalanche multiplication factor (M) has not been reported. In this article, an analytical empirical formula of multiplication factor for 4H-SiC p-n junction was presented based on accurate theoretical methodology. Utilizing the accurate impact ionization model, the impact ionization integrals (I) of electron and hole for a 4H-SiC abrupt diode were calculated by MATLAB. The simulations were carried out by MEDICI to verify the theoretical results. Breakdown voltage (BV) as a function of doping concentration was established. Ulteriorly, the multiplication factor as a function of doping concentration and reversely-biased voltage was derived in an empirical form. The research on the relative error analysis on I indicates that a tiny relative error of the electric field could lead to a large relative error of I. For a wide range of BV, the empirical formula could achieve a high accuracy for VR larger than 0.65BV with relative error less than 5%.