Phaseshifting phase measurement is studied according to the holographic principle and method, a function of synchronous superposition of object complex amplitude reconstructed from Nstep phaseshifting through one integral period is proposed, and Nstep phaseshifting function is for short. Meanwhile, a novel error analysis and maximum error evaluation method for phaseshifting phase measurement is presented. In Nstep phaseshifting phase measurement, the interferograms are seen as a series of inline holograms without diffraction and the reference beam is an ideal parallelplane wave. So the Nstep phaseshifting function can be obtained by summing after multiplying the interferogram by the original reference wave. In the ideal condition, the proposed method is a kind of synchronous superposition algorithm in which the complex amplitude is separated, measured and superposed. When error exists in measurement, the result from Nshifting function is the optimal expected value of the leastsquares fitting algorithm. In the above method, N+1step phaseshifting function can be obtained from Nstep phaseshifting function, indicating that the Nstep phaseshifting function can be separated into two parts: the ideal Nstep phaseshifting function and its errors. The phaseshifting errors in Nstep phaseshifting phase measurement can be treated as same as the errors of amplitude and intensity under understanding of N+1step phaseshifting function. The difficulties of the error evaluation in phaseshifting phase measurement are depressed by this error evaluation method. Meanwhile the maximum error evaluation method of phaseshifting phase measurement and its formula are proposed.