The simulated and measured electric field distributions along the separated function radio frequency quadrupoles(SFRFQ) are presented in this paper. Based on the simulations, the parameters of flake-electrode and the effective gap voltage gain have been optimized. As a design example, one set of SFRFQ electrode with 5 periods and inter-electrode voltage 70 kV is designed to accelerate O+ from 1MeV to 1.5MeV.