A shift control system for an automatic transmission comprises a shift valve for an auxiliary gear train and an accumulator for low gear train. The shift valve includes a spool which is movable between a first position where the hydraulic fluid is supplied to a first passage and hydraulic fluid is discharged from a second passage, and a second position where the hydraulic fluid is supplied to the second passage and hydraulic fluid is discharged from the first passage. The accumulator includes a working pressure chamber and a back pressure chamber which is connected with a third passage or engine brake passage.