The invention belongs to the technical field of photomultipliers, particularly relates to a photocathode, and discloses a high-sensitivity K-Na-Cs-Sb reflective multi-alkali photocathode and a preparation method and system thereof. The alkali source assembly comprises three alkali sources, namely a K source, a Na source and a Cs source. The alkali source assembly is connected with the exhaust table through a tail pipe to obtain an ultrahigh vacuum degree. The side window type photomultiplier comprises a glass shell, a cathode and a plurality of multipliers. The cathode and the multipliers areconfigured to be metal sheets with certain radians, and the inner surfaces of the cathode and the multipliers are plated with metal antimony layers. The side window type photomultiplier is connected to the alkali source assembly through an exhaust tail pipe. The corresponding alkali source applies current to realize self-heating, and releases alkali metal vapor to react with the antimony layer toform a multi-alkali cathode structure. The introduction process of K, Na and Cs is monitored by monitoring changes of leakage current and photocurrent, the introduction amount of K, Na and Cs is accurately controlled, and the problem that the cathode sensitivity is low due to the fact that too much or too little K, Na and Cs are introduced is solved.