The net thrust of engine is of great significance for aircraft performance calculation, engine power management and engine performance evaluation etc. Due to the impossibility to measure in-flight thrust of propulsion system directly, sensors must be installed into the engine interior flow paths to collect performance data, then the in-flight thrust can be calculated. The method of determining in flight thrust of civil aircraft powerplant is studied, and a formula for the net thrust of installed engine is provided. The process of thrust calculation was introduced, and the installed net thrust of a new high bypass ratio turbofan engine is calculated by using the data of engine performance flight test，and the comparison and analysis are made with the reference value. This study can provide reference for commercial aircraft design.