A novel method of generating piecewise-uneven fringes was proposed in order to get even fringes on the reference plane in oblique projection grating measuring system. The relationship between image pixels and phase was described by a piecewise function. To apply the fringes in double-frequency unwrapping, the method was further improved. Continuous unwrapping image series were obtained by left/right phase shifting which is a multiple of π. Double-frequency piecewise-uneven fringes method can be used for double-frequency unwrapping. The applications in piecewise-uneven fringes generation shows simplicity and good performance than conventional solutions. Contrast tests on arch profilmetry were taken herein. The results indicate that the measurement precision of piecewise-uneven fringes can get effectively improved compared with the method of even fringes.