The critical behaviors of the linear and higher-order nonlinear dielectric constants of ferroelectric liquid crystals (FLCs) are studied in the SmA phase near the SmA-SmC* phase transition. The critical exponents of the linear, third-order and fifth-order dielectric constants experimentally obtained are 1, 4 and 7, respectively. These exponents and the signs of nonlinear dielectric constants are in good agreement with those calculated theoretically from the phenomenological theory of Landau type. The influence of the order of phase transition (first or second) on the critical behavior of FLCs is also studied.