The invention discloses a determining system and a determining method based on key parameters of a streamer-leader system development model. The determining method comprises the steps of 1, measuring an average field intensity EL of a leader channel and an average field intensity Es of a streamer channel; 2, calculating a potential distribution Us after changing of all spatial charges and a background potential according to the EL and Es; 3, calculating streamer spatial newly added charges deltaQ according to the Us and the Es; and 4, calculating leader current IL according to the newly added charges deltaQ, and calculating a development velocity vL according to a parameter qL. The determining method and the determining system can enlarge model application range and improves calculation speed.