Owing to the PTD effect in the lenses of spatial filter, large temporal front distortion of femtosecond pulses occurs in the spatial filter. In this work, in order to design a spatial filter which is applicable to high power femtosecond laser system, PTD of achromatic doublet or separated doublet is calculated and special spatial filter is discussed in which achromatic doublet or separated doublet replace the single lens. PTD of the special spatial filter is little so temporal front distortion of femtosecond pulse in laser system is eliminated or reduced.