A model of double-layered microchannel heat sink with porous side fins is developed. Taking minimizing the maximum thermal resistance as the optimization objective and the aspect ratio of heat sink cell as the optimization variable, the constructal optimization was carried out for the heat sink with porous side fins under the constraints of given total volume of heat sink and volume proportion of fluid region. The effects of inlet velocity of coolant, porosity, the height ratio of upper channels to below channels, the volume proportion of fluid region and rib thickness ratio on the optimal constructs are analyzed, respectively. The results show that, with given the initial conditions, the maximum thermal resistance is 21.19% lower than its initial value after the aspect ratio is optimized. Decreasing the porosity is beneficial to reduce the maximum thermal resistance when the aspect ratio is smaller, while there is an optimal porosity to make the maximum thermal resistance minimize at a large aspect ratio. The changes of height ratio of upper channels to below channels and rib thickness ratio show few effects on the optimal construct.