Frequency-domain modeling is a crucial part of full-waveform inversion. Because of the huge computation problem it represents, several techniques have been applied to reduce the number of grid points per wavelength; among them, a rotating coordinate system has been widely used. However, scholars using rotating coordinate systems did not obtain solutions under rectangular grid conditions, which are extremely common in real data. Based on the average-derivative method, we have developed a new finite-difference scheme that used a 17-point operator to approximate spatial derivatives and the mass acceleration term. The new method can be applied to square and rectangular grids. After optimizing, the number of grid points per shortest wavelength was reduced to less than three with phase velocity errors no larger than 1%. To suppress the reflection from the boundary, we applied perfectly matched layer boundary conditions. Numerical tests further proved the validity of our adaptable 17-point scheme.