The invention provides a single cell capacity estimation method in a lithium ion battery pack. The implementation of the single cell capacity estimation method in the lithium ion battery pack is basedon a charging process of the lithium ion battery pack and a discharging process of the single cell; and N battery cells in the same aging state are contained in the lithium ion battery pack. The single cell capacity estimation method in the lithium ion battery pack comprises the following steps: taking a first fully charged single cell in the lithium ion battery pack as a reference battery, and calculating an approximate Q-OCV curve QV0 according to a charging and discharging curve; calculating the approximate Q-OCV curve QVi of an i-th cell whose capacity is to be estimated according to thecharging and discharging curve of the i-th cell whose capacity is to be estimated; performing differential calculation on the QV0 and the QVi separately to obtain capacity differential curves D0 and Di, normalizing the D0 and the Di, and translating the Di to coincide with the D0; recording the approximate OCVi of the charging cut-off time (the end point value of the curve) in the Di; calculatingan approximate SOC-OCV curve S0 of the reference battery according to the curve VQ0; determining a SOCi value corresponding to the OCVi in the curve S0; and calculating the actual capacity of the i-thsingle cell according to the partial discharging capacity of the i-th single cell and the SOCi value.