It is focused on CE-4 lunar orbit duel micro satellites mission for super-long wave astronomy. Under the strict resource constraints, the lunar orbit micro-nano satellites have very limited communication bandwidth. In order to maximize valuable scientific data, a method of in-orbit signal preprocessing was presented: the digital down conversion method was used to build the ultra-narrowband filter, which significantly reduces the data rate. The ultra-narrowband filtering scheme can perform comb filtering by increasing the down-conversion filter channels. The ultra-narrowband filter scheme simulations on the field programmable gate array show that the key technical indicators such as flatness and linear phase meet the requirements, and the data rate can be reduced by more than 10³ times compared with original data rate under the circumstances of the sampling rate is 80 MSPS with baseband bandwidth is 1 kHz.