In the past，stability evaluation of engineering rockmass is often based on the distribution of stress and deformation in engineering rockmass. Some scholars have put forward the concept of local stability factor of rockmass and its calculating method. But its condition is too harsh. The concept of is put forward and its formula is deduced based on the rockmass mechanics principle and strength criteria. The modified local stability factor has been used to evaluate the stability of surrounding rockmass of tunnel.