To obtain the output intrinsic safety criterion of Buck converters, the experimental research on its output-short circuit discharge characteristics is conducted by using the safety spark test apparatus. It is found that its spark discharge process can be divided into four stages, i.e., the dielectric-breakdown, spark-generation, spark-keeping and spark-extinguishment. According to the obtained spark discharge characteristics, the output short-circuit spark discharging energy (OSSDE) is deeply analyzed. It is indicated that the OSSDE of the Buck converter with a given inductance is a concave function of load resistance RL when RL is less than the critical resistance RLC corresponding to this inductance, while that is a convex function of RL in the case of RL > RLC. Considering the actual parameter range of the Buck converter, it is further pointed out that when RL RLC, the OSSDE increases with the increase of RL and reaches its maximum in the case of RL=RLC; when RL > RLC, the OSSDE first increases and then decreases with the increase of RL and reaches its maximum in the case of RL, DCM. The most dangerous operating conditions of the converter in whole dynamic range are obtained, i.e., the converter operates in DCM when the input voltage is the highest and RL=RL, DCM. Meanwhile, the maximum OSSDE is achieved. According to the energy equivalence, an output intrinsic safety criterion for Buck converter is proposed by modeling the output short-circuit discharging behaviour as a simple capacitive circuit. The theoretical analysis and proposed criterion are verified by the simulation and experimental results.