The invention, which belongs to the technical field of inertial navigation, discloses a rough alignment method of a strapdown inertial navigation system under an unknown latitude condition. The methodcomprises: collecting output data of a strapdown inertial navigation accelerometer and a gyroscope within alignment time and carrying out average filtering, and acquiring an average observed quantityvalue of the gravitational acceleration and an autoroatation angular velocity of the earth under a carrying system; calculating the orthogonal base projection of each axial unit vector of a navigation coordinate system in a carrier coordinate system by using the average observed quantity value; and then establishing an orthogonal attitude matrix by using an orthogonal base projection vector. According to the invention, the attitude matrix can be calculated directly based on the analytical solution without latitude estimation. After alignment completion, no additional orthogonalization processis needed. The example analysis shows that the alignment precision of the method is equivalent to that of the traditional alignment method, the alignment solution amount is reduced substantially, andthe alignment efficiency is improved.