The invention discloses a variable working condition type primary rocket system of a rocket based combined cycle engine. The variable working condition type primary rocket system comprises a propellant supply system and a primary rocket engine thrust chamber which are connected through a pipeline, wherein the propellant supply system comprises an oxidant supply system and a fuel supply system, the oxidant supply system comprises an oxygen flow rate control system, the oxygen flow rate control system comprises a plurality of oxygen conveying pipelines, each oxygen conveying pipeline is provided with an oxygen pressure relief valve for opening and closing the corresponding oxygen conveying pipeline, an outlet of each oxygen conveying pipeline is connected with an oxygen perforating plate, the fuel supply system comprises a plurality of fuel conveying pipelines, an outlet of each fuel conveying pipeline is provided with a cavitation venturi pipe, and an inlet of each fuel conveying pipeline is connected with a nitrogen pressure relief device. The variable working condition type primary rocket system of the rocket based combined cycle engine has the advantages that the flow rate of a propellant can be accurately, controllably, stably and quickly adjusted, the complexity is low, and the operation is simple.