The charge transfer in YBa2CU3O7-y system have been analysed by bond valence sum calculation. It is found that there exists an additional effective charge-transfer between the CuO2 planes and the CuO chain which varies nonlinearly with the oxygen stoichiometry and can be expressed as q = 0.151 exp(∼0.007/y4) + 0.35 exp(0.5 – 0.5/y4). This nonlinear charge-transfer is associated with the structural transitions of orthorhombic (I)-to-orthorhombic(II) and orthorhombic(II)-to-tetragonal symmetry, and thus related to the oxygen ordering in the CuO chains. A new relationship between the mobile hole number P(2) the localised hole number P(l) and the total hole number P has been established as P(2) = P - P(1), P(1) = 0.5 - y - q. The results also show that the 90 K and 60 K plateaus in Tc as a function of oxygen stoichiometry correspond to the plateaus in the curve of q versus y, indicating that both the nonlinear variations of Tc and the charge-transfer with the oxygen stoichiometry are intrinsically correlated.