The invention relates to an on-site testing device and a determination method of explosiveness of mixed gas. The device comprises an automatic gas sampling system, an explosion cavity body, a fire retardant pressure release system, a superhigh temperature ignition system, an intelligent control and data acquisition and analysis system and a power supply that are arranged in a box body. The automatic sampling system automatically sucks mixed gas on site into the explosion cavity body; the superhigh temperature ignition system performs ignition testing on the unknown mixed gas in the explosion cavity body; pressure and temperature variations in the explosion cavity body are determined for quick explosiveness judgement; and double judgement indices of increments of maximum explosion pressure and maximum temperature in a burning and explosion process are provided. The testing device can achieve on-site quick explosiveness determination of the unknown mixed gas, is safe and reliable, and easy and simple to operate and can provide experimental decision bases for emergency treatment of a flammable and explosive gas leakage field.