By Fourier optics method,fiber-optic confocal sensor using a gradient-index(GRIN) lens has been analyzed theoretically as an imaging system upon the weak object approximation.The imaging mechanism to measurement depth structure is analysed,which can be described precisely by a coherence transfer function(CTF),because fiber-optic have eigen mode and coherent to the response of optical amplitude and phase.It has been proved that the fiber-optic confocal sensor system behave as a linear shift invariance system,performs like a lowpass filter and has high axial resolution.