The Tibet-ASgamma experiment has a water-Cherenkov muon-detector array (MD) as a part of 3 different arrays with specific purposes. To increase the ability of getting the primary proton, helium and iron spectra up to 10^16 eV and clarifing their spectral "knee" positions, we need to upgrade current PMT capabiltty in each MD cell. The current dynamic range of the PMTs is 5 to 2000 photoelectrons (PEs) which is enough for the purpose of observation of high-energy celestial gamma rays. However, according to Monte-Carlo simulations, we need a dynamic range of 100 - 10^6 PEs for the knee problem. For this end, we have a plan of adding two 8-inch-in-diameter PMTs in
each MD cell. We have compared the characteristic features between R5912-PMT made by Japan Hamamatsu and CR365-PMT made by Beijing Hamamatsu, and found that CR365-PMT fulfill our requirements. In this paper, test results of the first batch of 20 CR365-PMTs made by Beijing Hamamatsu for the MD array are shown.