BrightK-shell X-ray emissions are generated through the interaction between intense femtosecond laser pulses and Ar clusters. The total yield ofK-shell X-ray photons reaches 1 1011 photons/shot with a conversion efficiency of 2.8 10-5 in 4. When Ar clusters are irradiated by a laser pulse with intense prepulse, the yield ofK-shell X-ray photos will decrease due to prepulse ionization. Keeping high-density plasmas interacting with the main pulse is essential for obtaining the highest X-ray yield.