High-flow　aeroengine　transient　tests　involve　strong　coupling　and　external　disturbances,　which　pose　significant　challenges　for　intake　environment　simulation　systems　(IESSs).　This　study　proposes　a　compound　control　scheme　that　combines　fixed-time　active　disturbance　rejection　with　static　decoupling　methods.　The　scheme　integrates　a　fixed-time　sliding-mode　controller　(FT-SMC)　and　a　super-twisting　fixed-time　extended-state　observer　(ST-FT-ESO).　A　decoupling　transformation　separates　pressure　and　temperature　dynamics　into　two　independent　loops.　The　observer　estimates　system　states　and　total　disturbances,　including　residual　coupling,　while　the　controller　ensures　fixed-time　convergence.　The　method　is　deployed　on　a　real-time　programmable　logic　controller　(PLC)　and　validated　through　hardware-in-the-loop　(HIL)　simulations　under　representative　high-flow　scenarios.　Compared　to　conventional　linear　active　disturbance　rejection　decoupling　control　(LADRDC),　the　proposed　scheme　reduces　the　absolute　integral　error　(AIE)　in　pressure　and　temperature　tracking　by　71.9%　and　77.9%,　respectively,　and　reduces　the　mean-squared　error　(MSE)　by　46.0%　and　41.3%.　The　settling　time　improves　from　over　5　s　to　under　2　s.　These　results　demonstrate　improved　tracking　accuracy,　faster　convergence,　and　enhanced　robustness　against　disturbances.