The binary properties of open clusters place crucial constraints on star formation theory and clusters dynamical evolution. We develop a comprehensive approach that models the color-magnitude diagram (CMD) of the cluster members as the mixture of single stars and photometric unresolved binaries. This method enables us to infer the binary properties, including the binary fraction $f_mathrm{b}$ and binary mass-ratio distribution index $gamma_q$ when a power-law is assumed, with high accuracy and precision, which were unfeasible in conventional methods. We employ a modified Gaussian process to determine the main sequence ridge line and its scatter from the observed CMD as model input. As a first example, we apply the method to the open cluster NGC3532 with the Gaia DR2 photometry. For the cluster members within a magnitude range corresponding to FGK dwarfs, we obtain $f_mathrm{b} = 0.267pm0.019$ and $gamma_q = - 0.10pm0.22$ for binaries with mass ratio $q > 0.2$. The $f_mathrm{b}$ value is consistent with the previous work on NGC3532 and smaller than that of field stars. The close to zero $gamma_q$ indicates that the mass ratios of binaries follow a nearly uniform distribution. For the first time, we unveil that the stars with smaller mass or in the inner region tend to have lower $f_mathrm{b}$ and more positive value of $gamma_q$ due to the lack of low mass-ratio binaries. The clear dependences of binary properties on mass and radius are most likely caused by the internal dynamics.
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