In order to accurately diagnose the electron density of laser-produced plasma, the method of relative intensity ratio of helium-like resonance to combination lines was proposed based on the polarization spectrum. It took into account polarization characteristics of the X-ray in laser-produced plasma, modified the real intensity ratios of the resonance to intercombination lines of He-like ions, then derivated the density of plasma. The experiment was carried out at the 2×10 J laser facility at Laser Fusion Research Center of, China Academy of Engineering Physics (CAEP). X-ray spectra emitted from the aluminum plasmas were recorded by polarized PET(002) crystal spectrometer. By correcting two Al helium-like spectra, it was calculated that the electron density was about 1.5×1 020 cm－3. The experimental results showed that the arithmetic was adapted to diagnose laser-produced high temperature high-density plasmas.