he density roll-over dependence on H-mode power threshold is observed on EAST for the first time.Shots in campaign 2014 and 2015 with lower hybrid wave heating and a toroidal magnet field of 2.35 T have observed roll-over dependence in density region of (n)e =2.0~3.5× 1019 m-3, while shots with 1.79 T and 1.9 T exhibit linear dependence consist with scaling law.The minimum density for accessing H-mode on EAST is (n),min/nG ≈ 0.38 in BT=2.35 T and (n)e,min/nG < 0.25 in BT =1.79~1.9 T, which is sensitive to toroidal field and better described in normalized density.For comparison,(n)e,mm ≈ 4× 1019m-3 in ASDEX-Upgrade[1]with ECH in both D and He discharges (Ip=1.0 MA, BT=2.3 T).The normalized minimum density of ASDEX-Upgrade is close to that of EAST with similar magnetic field.Both the experimental result in JET[2] with ITER-Like-Wall and prediction result in so called "FM3"model[3] reveal that, the minimum density increases roughly linearly with magnetic field ne, min∝BT0.8.Besides, H-mode power threshold have a good correlation with outer leg length (distance from X-point to outer strike), which explains the data scattering in the same density and toroidal field.Normalized power threshold increases with the increase of X-point height,consistent with results in JT-60U[4] that power threshold is lower in W-shaped divertor than open divertor (in the formal case the distance from X-point to divertor floor is shorter) and DⅢ-D[5].Normalized power threshold decreases with increase of X-point radius, consistent with results in NSTX[6].Yet outer leg length (consist of contribution from both X-point height and radius) have the greatest correlation with normalized power threshold, suggesting outer leg length is a deeper control variant.Supersonic molecule beam injection inducing multi-L-H transitions have been studied via Microwave Reflectometry.It is observed a steady increase of edge density and density gradient happen in the vicinity of separatrix and then shift inward to pedestal in 30 ms prior to the L-H transition.SMBI fueling is related and may provide extra trigger via increasing density gradient, which requires further confirmation.This behavior is comparable to pellet injection in trigging H-mode experiment in Ⅲ-D[7].