Variable temperature method was lent for low temperature Raman study on bulk monocrystal 6H-SiC in the temperature range from 80 to 320 K. Some Raman folding modes of 6H-SiC were assigned in the spectrum. The peak position and line width of optical phonon A1(LO) versus temperature mainly below RT was focused on by 3-phonon and 4-phonon models. The result showed that as the temperature decreased the line width decreased, while the peak position shifted to high wave number. It was found that as the temperature varied below 160 K, the change in peak position and line width was little, different from it was at RT, revealing that the line type of A1(LO) mode was mainly decided by the phonon characteristics and the effect of temperature could be ignored when it was below 160 K. It was showed that 4-phonon model was closer to the experiment data for fitting. Both the third and forth anharmonic vibration contributed to the spectrum, while the former was the main process. Moreover, the phonon lifetime became longer when the temperature fell because of the decrease in the atomic thermal motion.