A construction machine starting assist system, comprising: a) an engine of a construction machine; b) an input unit configured to receive a key-on signal and a key-off signal of the engine; c) a hydraulic pump which is operated by the engine; d) an actuator which is operated by hydraulic oil discharged from the hydraulic pump; e) a regeneration valve which is switched so that a part or an entirety of the hydraulic oil returned from the actuator and a part or an entirety of the hydraulic oil discharged from the hydraulic pump are supplied for regeneration; f) an accumulator configured to be charged with the hydraulic oil supplied from the regeneration valve; g) a charging valve configured to discharge the hydraulic oil from the accumulator when the key-on signal is inputted into the input unit; h) a hydraulic motor which is operated by the hydraulic oil discharged from the charging valve, and is connected to the engine and configured to assist in starting the engine; i) a solenoid valve configured to adjust a swash plate angle of the hydraulic motor; j) a controller configured to control the regeneration valve and the solenoid valve; characterised in that the solenoid valve is configured to receive the hydraulic oil from the accumulator and the system further comprises k) a pressure sensor configured to detect a hydraulic pressure of the hydraulic oil with which the accumulator is charged, wherein l) the controller determines the hydraulic pressure of the hydraulic oil with which the accumulator is charged by the pressure sensor when the input unit receives the key-off signal, when the hydraulic pressure of the accumulator is below a predetermined value, the controller operates the engine to charge the accumulator with the hydraulic oil discharged from the hydraulic pump until the hydraulic pressure of the hydraulic oil with which the accumulator is charged reaches the predetermined value, and then the controller turns off the engine, and when the hydraulic pressure of the accumulator is equal to or greater than the predetermined value, the controller turns off the engine after receiving the key-off signal. a) an engine of a construction machine; b) an input unit configured to receive a key-on signal and a key-off signal of the engine; c) a hydraulic pump which is operated by the engine; d) an actuator which is operated by hydraulic oil discharged from the hydraulic pump; e) a regeneration valve which is switched so that a part or an entirety of the hydraulic oil returned from the actuator and a part or an entirety of the hydraulic oil discharged from the hydraulic pump are supplied for regeneration; f) an accumulator configured to be charged with the hydraulic oil supplied from the regeneration valve; g) a charging valve configured to discharge the hydraulic oil from the accumulator when the key-on signal is inputted into the input unit; h) a hydraulic motor which is operated by the hydraulic oil discharged from the charging valve, and is connected to the engine and configured to assist in starting the engine; i) a solenoid valve configured to adjust a swash plate angle of the hydraulic motor; j) a controller configured to control the regeneration valve and the solenoid valve; characterised in that the solenoid valve is configured to receive the hydraulic oil from the accumulator and the system further comprises k) a pressure sensor configured to detect a hydraulic pressure of the hydraulic oil with which the accumulator is charged, wherein l) the controller determines the hydraulic pressure of the hydraulic oil with which the accumulator is charged by the pressure sensor when the input unit receives the key-off signal, when the hydraulic pressure of the accumulator is below a predetermined value, the controller operates the engine to charge the accumulator with the hydraulic oil discharged from the hydraulic pump until the hydraulic pressure of the hydraulic oil with which the accumulator is charged reaches the predetermined value, and then the controller turns off the engine, and when the hydraulic pressure of the accumulator is equal to or greater than the predetermined value, the controller turns off the engine after receiving the key-off signal.