A novel wavelength conversion method for wavelength division multiplexing(WDM) n ets by use of cascaded second-order nonlinearities (χ(2)∶χ(2) ) in D-fiber grating after periodically thermal/electric-field poling is p roposed,and is theoretically analyzed for the first time.The coupled mode equati ons of the χ(2)∶χ(2) nonlinearities are derived,and the analytic expressions for the electric-field intensity amplitude of the conversi on light waves and conversion efficiency η are obtained under the small signal condition,which are well consistent with numerical calculations.Both analytic ex pressions and numerical results show that,under the suitabl phase-matching condi tion,η is proportional to the logarithm of the square of pump light power,and the 4th power of the grating length L & second-order nonlinearity d.The calculat ed results also show that the wavelength conversion efficiency η and conversion bandwidth Δλ of over-17dB and 120nm respectively can be reached with this fib er grating.With the increase of L,η increases quickly white Δλ decreases quic kly.The results of simulated calculation and theoretical analysis show that the cascaded χ(2)∶χ(2) process is different from that of qu asi-phases-matched difference-frequency-generation.