For a rectangular thin plate with different support conditions of four edges， a double Fourier series with additional terms was taken as the deflection function， and linear algebraic equations for solving the undetermined coefficients were derived. A unified structural calculation formula was obtained for the plate with any combination of simply supported， clamped， and free edges. The non-convergence of a series solution of bending moment at a point where a concentrated load is applied was discussed. In addition， the aspect ratio problem that needs to be achieved when simplifying a two-way plate is also discussed. The results show that the bending moment at the point can be calculated through the finite difference method using the deflection value， where a differential step length of 10 mm was selected. A plate with two opposite edges supported and two other edges free， and a plate with one edge clamped and the other three edges free， can be taken as a one-way plate. A plate with all four edges supported can be treated as a one-way plate with two ends clamped， one end simply supported and the other end clamped， or two ends simply supported if its aspect ratio reaches 2∶1， 2.5∶1， or 4.5∶1， respectively. A plate with three edges supported and the last edge free can be seen as a one-way plate with two ends clamped （and one end simply supported and the other end clamped）， or two ends simply supported if its aspect ratio gets 1∶1 or 2∶1， separately； this type of plate can also be simplified as a cantilevered one-way plate at an aspect ratio of no less than 6∶1. A plate with two adjacent edges supported and the other two edges free can be taken as a cantilevered one-way plate if its aspect ratio reaches 2∶1 when the two supported edges are all clamped， or if the aspect ratio reaches1.5∶1 when one edge is clamped and an adjacent edge is simply supported.