High-efficiency terahertz(THz)wave generation with multiple frequencies by optimized cascaded difference fre-quency generation(OCDFG)is investigated at 100 K using a nonlinear crystal consisting of a periodically poled lithium niobate(PPLN)part and an aperiodically poled lithium niobate(APPLN)part.Two infrared pump waves with a frequency difference ωT1 generate THz waves and a series of cascaded optical waves in the PPLN part by cascaded difference fre-quency generation(CDFG).The generated cascaded optical waves with frequency interval ωT1 then further interact in the APPLN part by OCDFG,yielding the following two advantages.First,OCDFG in the APPLN part is efficiently stim-ulated by inputting multi-order cascaded optical waves rather than the only two intense infrared pump waves,yielding unprecedented energy conversion efficiencies in excess of 37％at 1 THz at 100 K.Second,THz waves with M timesωr1 are generated by mixing the mth-order and the(m+M)th-order cascaded optical waves by designing poling period distributions of the APPLN part.