The gravity vector measurement contains information about the gravity direction (i.e., the deflection of the plumb line), which is crucial for determining the geoid. To obtain the horizontal component of oceanic gravity disturbance vector, a two-step method based on the strapdown inertial navigation system/global navigation satellite system(SINS/GNSS) combined system is proposed for determining the horizontal component of the oceanic gravity disturbance vector. Firstly, the strapdown inertial navigation algorithm and Kalman filter are used to estimate and compensate for accelerometer bias and attitude angle errors. Then, based on the accurately estimated accelerometer bias and attitude angle errors, the calculation equation of the horizontal component of the gravity disturbance vector is presented. The model of the horizontal component of the gravity disturbance vector is described by using a second-order Markov random process and separated from the bias of the accelerometer. The forward-smoothing Kalman filter is used to accurately estimate the horizontal component of the gravity disturbance. The offline calculation of ship-borne test data shows that the internal coincidence accuracy of the horizontal component of the oceanic gravity disturbance vector is better than 2 mGal(1 Gal=1 cm/s2).