The characteristic of the change in wavelength of a long-period fiber gratings(LPFGs) attenuation band was analyzed with changes in external index of refraction in detail.Then,based on this characteristic of LPFGs,the designing of a new type of tunable optical filter was put forward.And it was calculated concretely using matrix method of analyzing piecewise-uniform LPFGs.It is concluded that this kind of optical filter has advantages of high precision,independently and continuously tuning of resonance wavelength and peak loss,and all-fiber in-line device.The tuning range of filter wavelength reaches 15 nm,and the tuning range of peak loss reaches 9 dB.