An approach is proposed to determine the phaseshifting value in phaseshifting digital holography. By employing a random phase plate behind the object, we successfully made the phase distribution of the object on the holography plane a spatially approximation random distribution. Then an algorithm utilizing this character to extract phaseshifting value is proposed. Computer simulations indicate that the extracted phaseshifting value is very close to the predetermined, and the relatively error is less than 0.1%. At the same time, by comparing the calculation results with and without the random phase plate, we find that the phase distribution of an object has some correlation after diffraction.