The photorefractive holographic grating in a Mn doped K1-yNayBa2-xSrxNb5O15 (KNSBN) is written with two 0.27mW He-Ne laser beams (632.6nm) and is read with one quasicontinuous dye laser beam (621.8nm, 1.6mW or less). It is found that the Bragg diffracton suddenly reaches its maximum at the beginning and then decays to its minimum with a time constant of about 0.6s. The ratio of the maximum to the minimum can be higher than 7.5. A theory including pyroelectric effect has been developed and it fits the experiment well.