The decay of virioplankton is one of the most important process of virioplankton population dynamics. Besides the solar-ultraviolet radiation b, other factors can also lead to the decay of virioplankton. Experiment was carried out to study the decay rate of cyanophage PP caused by some environmental factors: non-UVB radiation in sunlight, particle materials, heat-sensitive substances and water temperature. The results indicated that the decay rate of cyanophage PP by the non-UVB radiation part of sunlight at noon was 3.8%.h(-1), however, the decay rate was swiftly attenuated to less than 1.0%-h(-1) with the attenuation of sunlight intensity; in dark condition, the decay rate of cyanophage PP in field sample was steadily 2.5%-h(-1), among which 2.01%-h(-1) contributed to particle materials (in more turbid waters, the number would be as high as 15.0%-h(-1)), and 2.1 %-h(-1) contributed to heat-sensitive substances. The dark decay rate in field sample varied at different temperatures, it was 8.75%-h(-1) at 35 degrees C which was 2.5 times greater than the decay rate at 28 degrees C. Our results suggest that heat-sensitive particle substances are the most important contributing factor to viral dark decay in high eutrophic freshwater and also point to the significance of water temperature in influencing the dark decay rate.