As there is no uniform standard to measure the dose of myopia control defocus lens, a simple and fast theory model EDI (equivalent defocus index) for evaluation of the dose is proposed. The parameters of the myopia control defocus lens which affect the visual performance are investigated by simulations in the optical software, then the definition of EDI is given. Three typical myopia control defocus lenses are evaluated by EDI and compared with modulation transfer function (MTF), which indicates the validity of EDI.