A heat dissipation estimating method is disclosed. The heat dissipation estimating method includes following steps: providing an input heat to a fin of a heat sink unit; obtaining an average temperature of the fin according to the input heat; obtaining an output heat according to the average temperature; determining whether the input heat is the same as the output heat or not; while the input heat is different from the output heat, updating the input heat according to the output heat and repeating the above steps until the input heat is the same as the output heat; and while the input heat is the same as the output heat, obtaining a total heat dissipation value of the heat sink unit according to the input heat and a ratio value.