As the strain caused by the packaging process of a diode laser influences on the output power, wavelength and the reliability of the high power diode laser bar, this paper researches the measurement of packaging-induced stain in the laser. On the basis of the variation of polarization degree of the output lights in the diode laser can reveal the variation of band edges of the quantum wells in active region of the diode laser bar, it deduces the dependence of the polarization degree of laser outputs on the packaging-induced stain by using electro-luminescent spectroscopy. The strain in active region of a 800 nm GaAsP/GaInP high power diode laser bar is studied and the experimental results are in good agreement with the theoretical simulation. Furthermore, the experimental results are compared with the original strain in active region of the diode laser bar, and it is found that the bar is compressed by the copper heat sink during the packaging process, and the packaging strain is induced in the active region. The packaging-induced strain at the center of the laser bar is higher than the one at the edges. Moreover, the experimental strain of the bar shows more variation apparently, which can be considered as that the defects are created in the indium solder layer fabricated by electrolytic deposition method. In the experiment, the maximum strain is 1.370×10-3 and the defect density is 40.8%.Obtained results demonstrate that the polarization degree measurement of the diode lasers can reveal the defects and the packaging-induced strains of diode lasers accurately and can well evaluate their qualities.