The invention discloses a system and method for synchronizing a GPON OLT and a previous-level clock in a ToD service scene, relating to the field of optical fibre access networks. When the system and the previous-level clock are synchronous, a clock module is synchronous with a clock source of the GPON OLT; the previous-level clock respectively sends a 1PPS clock signal to the clock module and a ToD signal to the GPON OLT; the clock module performs synchronous calibration of a second counter and a nanosecond counter according to the 1PPS clock signal; after simultaneously detecting a sampling signal and a GPON frame starting time signal sent by the GPON OLT, the clock module records the starting time of a GPON downlink super-frame to be sampled; and the GPON OLT reads the starting time of a current GPON downlink super-frame from the clock module. The system disclosed by the invention can be precisely synchronous with the previous-level clock; the sending time corresponding to the starting bit of a specific GPON downlink super-frame can be precisely obtained; furthermore, the upgrading cost of the GPON OLT can be reduced; and the system and the method are applied to popularization.