Aimed at the positioning dependence of shipboard star sensor on external horizontal attitudes, a method of high precision horizontal attitudes based on the features of gravity in the Inertial coordinate system was put forward.Making use of the output of star sensor,the attitude transformation matrix in the carrier coordinate system was related to the one in Inertial coordinate system.The measurement information projection of the accelerometer,filtering by adaptive filter,was then converted in the Inertial coordinate system,which could eliminate external interference like the noise of wind and the waves or the accelerometer.At last the more pure gravity information was utilized to calculate high precision horizontal attitudes,which would be the external horizontal standard needed for star sensor positioning. The simulation results showed that this method could calculate the horizontal disturbance information of the ship effectively.The horizontal precision was high without error accumulation. This method increased the location precision of the star sensor, which could well meet the requirements of navigation precision of ship for long time sailing on the sea.