High-efficiency terahertz (THz) wave generation with multiple frequencies by optimized cascaded difference frequency 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 ω _T1 generate THz waves and a series of cascaded optical waves in the PPLN part by cascaded difference frequency generation (CDFG). The generated cascaded optical waves with frequency interval ω _T1 then further interact in the APPLN part by OCDFG, yielding the following two advantages. First, OCDFG in the APPLN part is efficiently stimulated 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 times ω _T1 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.