The invention discloses a composite phase mask, the phase distribution function of which contains two different functions, namely, a sinusoidal type function and an exponential type function. The function expression of the phase distribution function Theta(x, y) is as follows: Theta(x, y) is equal to Alpha question mark x question mark (Epsilon x Rho(Beta x <2> plus Epsilon x Rho(Beta y<2>)) plus(1/2)Alpha(sin(Omega x) plus sin(Omega y)), and in the expression, Omega question mark x question mark (Epsilon x Rho(Beta x<2> plus Epsilon x Rho(Beta y<2>)) is an exponential type function, and (1/2)Alpha(sin(Omega x) plus sin(Omega y)) is a sinusoidal type function; wherein Alpha expresses the amplitudes of the exponential type function and the sinusoidal type function, Beta expresses the modulation factor of the exponential type function, Omega expresses the angular frequency of the sinusoidal type function, and x, y is space coordinate with normalized aperture plane. The invention also discloses an imaging system which adopts the composite phase mask compounded by the sinusoidal type function and the exponential type function; and the imaging system has smaller numerical value of theFisher information, is not sensitive to defocusing, has better inhibiting effect on defocusing amount and has better expansion capability of depth of focus.