The　effects　of　duration　of　the　endurance　time　analysis　on　the　seismic　response　of　the　underground　structures　are　investigated.　The　typical　two-story　three-span　subway　station　embedded　in　the　sites　of　classes　II　and　III　is　used　as　the　prototype.　The　response　spectra　at　the　engineering　bedrock　is　used　as　the　target　ones　to　develop　six　endurance　time　acceleration　functions　as　the　input　motions　for　the　nonlinear　soil-structure　interaction　system.　Using　the　results　from　the　incremental　dynamic　analysis　as　the　reference,　the　effects　of　time　duration　of　the　endurance　time　analysis　on　the　seismic　analysis　of　underground　structures　are　significant.　A　formula　to　compute　the　optimal　time　duration　based　on　the　variations　of　earthquake　ground　motion　energy　measure,　Arias　intensity,　with　the　target　time　is　proposed　and　validated.　It　can　be　seen　from　the　numerical　results　that　endurance　time　acceleration　functions　with　durations　of　30　and　45　s　are　more　favorable　for　the　underground　structures　embedded　in　the　sites　of　classes　II　and　III,　respectively.　Moreover,　the　proposed　formula　for　the　optimal　duration　estimation　is　feasible　for　the　seismic　performance　evaluation　of　underground　structures　in　the　above　two　site　classes.　The　results　from　the　endurance　time　analysis　are　more　accurate　when　the　total　energy　in　the　artificial　endurance　time　acceleration　function　is　close　to　the　actual　earthquake　records.　©　2021,　Editorial　Office　of　Chinese　Journal　of　Geotechnical　Engineering.　All　right　reserved.