To　address　the　strong　decoupling　between　pressure　and　temperature　variables　in　the　intake　environment　simulation　system　（IESS）　of　altitude　test　facility，　as　well　as　the　significant　total　disturbance　caused　by　rapid　transient　tests，　a　novel　fixed　time　sliding　mode　controller　（FxTSMC）　with　a　fixed　time　extended　state　observer　（FxTESO）　was　designed，　and　thus，　a　fixed　time　active　disturbance　rejection　compound　decoupling　control　method　（FxT-ADRCDC）　was　proposed.　Specifically，　a　static　decoupling　method　was　adopted　to　decouple　the　IESS　into　two　single-input，　single-output　systems，　achieving　independent　control　of　intake　temperature　and　pressure.　In　terms　of　total　disturbance　estimation　and　suppression，　the　FxTESO　was　utilized　to　rapidly　and　accurately　estimate　the　states　and　total　disturbances　including　dynamic　coupling　to　deal　with　the　strong　disturbance，　and　then，　a　novel　FxTSMC　that　can　avoid　singular　problems　and　converge　rapidly　was　proposed，　the　total　disturbance　was　then　compensated　into　the　FxTSMC，　obtaining　the　FxT-ADRCDC　method.　The　Lyapunov　function　was　designed　to　prove　the　fixed　time　stability　of　the　closed-loop　control　system，　obtaining　the　upper　bound　of　the　settling　time，　which　does　not　depend　on　the　initial　conditions.　Finally，　the　transient　flight　mission　experiments　on　a　high-confidence　IESS　platform　were　launched　and　the　comparisons　between　the　FxT-ADRCDC　method　and　the　linear　active　disturbance　rejection　decoupling　control　（LADRDC）　method　were　carried　out.　The　results　show　that　the　absolute　integral　error　of　intake　pressure　and　temperature　under　the　FxT-ADRCDC　is，　on　average，　reduced　by　approximately　70.3%　and　70.7%，　respectively，　compared　to　the　LADRDC　method.　Moreover，　the　valve　swing　satisfies　the　requirements　for　practical　engineering.　The　proposed　method　effectively　achieves　high-quality　decoupling　control　of　intake　pressure　and　temperature，　and　provides　strong　technical　support　for　improving　the　system’s　rapid　and　accurate　dynamic　modelling　capabilities.　©　2025　China　Aerospace　Science　and　Industry　Corp.　All　rights　reserved.