In order to diagnose X-ray line polarization of laser-produced plasmas, two crystal spectrometers have been used for measuring spectra. They can measure X-ray lines radiated by plasmas in directions of vertical and horizontal for detection of polarization degree. Measuring principle of X-ray polarization is studied in detail and mathematical computation is given. PET is dispersion crystal of spectrometer and X-ray spectra emitted from planar Al target are photographed by using spectrometers with two pieces of image plates (15 mm×35 mm) on 2×10 J laser facility. Experimental result shows that the spectra of He-like and Li-like in directions of vertical and horizontal can be obtained. Polarization degree of resonance line w is calculated based on intensity rate of resonance line w and intercombination line y. The relation between laser energy and X-ray polarization degree is found with experimental results of two shots and effect of polarization degree on diagnosis of plasma temperature is discussed.