Based on the introduction of traditional flatness error analysis method, this paper introduces a new method using the minimum two-ride fitting technique to propose a new method for the analysis of the coplanarity of marine steel structures. The points are evenly collected on the surface, and the collected points are fitted to an ideal reference plane by least squares method. Finally, the vector distance from each measurement point to the ideal reference plane is calculated, so as to accurately analyze the coplanar surface of the surface to be measured. The coplanarity analysis method introduced in this paper can be extended to the coplanarity analysis of any plane, including the calculation of the coplanarity of the FPSO generator base support plate and the coplanarity analysis of the marine steel structure blind flange.