The photo-thermo-refractive (PTR) volume grating is formed by PTR effect in photo-sensitive glass. PTR volume holographic grating has unique advantages for the applications of laser technologies owing to its excellent wavelength and angular selectivity, high diffraction efficiency, high thermal stability and high damage threshold. The diffractive characteristics and stability of the photo-thermo-refractive volume grating are investigated. A transmission hologram is recorded under 325 nm laser exposure in the PTR glass. According to the principle of variable wavelength readout, experimental setup is designed for readout of transmission photo-thermo-refractive volume grating. The angular selectivity and the stability under laser irradiation of the transmission PTR volume grating after thermal development are measured experimentally. The photo-saturation effect is observed and a characteristic time constant is acquired through by fitting the calculation and experimental results to describe the stability.