The invention discloses an offshore platform top-loading type draft measurement device and an FPSO offshore platform. The offshore platform top-loading type draft measurement device comprises a waveguide, an ultrasonic radar liquid level sensor is installed at the top end of the waveguide, a partition ball valve is installed at the bottom end of the waveguide, and the partition ball valve is connected to a hand wheel through a small shaft transmission mechanism. The FPSO offshore platform comprises the offshore platform top-loading type draft measurement device. According to the technical scheme, the offshore platform top-loading type draft measurement device and the FPSO offshore platform have the following advantages that a worker performs all operations and maintenance on a deck without entering an airtight cabin, and operation convenience and safety are improved; no instrument, cable or other measuring device exists in the cabin, the maintenance requirement is greatly reduced, and the offshore platform top-loading type draft measurement device is free of maintenance within the service period of the offshore platform; by adoption of a high-precision narrow-beam ultrasonic radar, the scattering range is small, the measurement precision is high, and the measurement error is only 0.03%; and a sea valve can be manually closed in emergency circumstances, and safety of the offshore platform is ensured.