Compared with traditional optical microscopy, digital holographic microscopy can be used to measure phase information of samples. Therefore it allows quantitative detection and effective observation of transparent biological samples such as living cells. But the field curvature caused by the microscopic imaging in the hologram recording process will affect phase distribution of the reconstructed image. In order to correct the field curvature, a phase-subtraction method is presented. Recording two holograms before and after inserting the sample, unwrapping the numerical reconstructed images of the two holograms solely and subtracting them each other, the corrected phase distribution of the sample image will be obtained. As examples, some cicada wing and epidermal cells of garlic are measured by using digital holographic microscopy and the field curvature is corrected by numerical-correction method and phase-subtraction method, respectively. It is shown that the phase-subtraction method is more credible and convenient to get three-dimensional phase information and more effective to correct the field curvature than current numerical-correction method in digital holographic microscopy.