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 a mixture of single stars and photometric unresolved binaries. This method enables us to infer the binary properties, including the binary fraction fb and binary mass ratio distribution index ?q when a power law is assumed, with high accuracy and precision, which was 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 NGC 3532 with the Gaia DR2 photometry. For the cluster members within a magnitude range corresponding to FGK dwarfs, we obtain fb=0.267±0.019 and ?q =-0.10±0.22 for binaries with mass ratio q > 0.2. For the first time, we unveil that the stars with smaller mass or in the inner region tend to have lower fb and more positive value of ?q due to the lack of low q binaries. The dependences of binary properties on mass and radius are caused by internal dynamics.