For the need of X-ray wavelength determination of laser plasma, based on the auxiliary diaphragm method, a novel method was presented for the wavelength determination of the X-ray spectral lines measured with planar crystal spectrometer in theory. We develop an improved method to determine the distance from the crossing line of crystal and recording plane to the first auxiliary diaphragm, and curvature of spectral line is used to determine the angle between crystal and recording plane. In normal auxiliary diaphragm method, reference line is used to determine the two parameters. However, we can obtain wavelengths for all recording spectral lines without any reference lines by using this method. In practical use, the precision of wavelength determination can achieve 1×10-3 nm by increasing the distance between two diaphragms and adjusting the relative position between crystal and recording plane.