The curve of cosmic ray energy spectrum is turned around PeV region, which is called "the knee" of the cosmic ray spectrum. The origin of the PeV knee of cosmic ray spectrum remains a puzzle since its first discovery near 70 years ago. Searching for the origin of the knee is one of main aims of LHAASO experiment, a hybrid cosmic ray observatory, which is building on Haizi mountain, south of China. In this talk, the puzzle of the knee of cosmic ray spectrum will be introduced briefly together with the LHAASO experiment introduction. Then I will focus on the technique of scintillator detector of LHAASO and the photomultiplier tube used in this detector. The PMT test bench of Shandong University will also will be introduced.