Low-frequency (<50 GHz) line surveys have unique advantages in searching for complex organic molecules (COMs) and radio recombinations lines (RRLs). The low-frequency radio equipments is continuously increasing both in and outside China (e.g., TMRT, FAST, ALMA band 1, SKA), demonstrating the eager demand of low-frequency spectral templates for further radio spectroscopic studies. However, so far, deep line survey covering the whole low-frequency band is still lacked. Motivated by this, we maintain an on-going sensitive (in mK scale) multi-band (1-50 GHz) TMRT radio line survey towards Orion KL. More than one thousand line features are detected from the already accomplished Ka/Q-band (26-50 GHz) pioneer surveys, including many previously undetected COM lines and RRLs. Especially, RRLs of carbon/oxygen ions are detected in the interstellar medium for the first time. This finding greatly expands the scope of our understanding of ion RRLs, and makes it possible to determine elemental abundances in photoionized nebulae, especially in places where high extinction makes it hard or impossible to do so with visible emission lines. We expect the TMRT survey, in accompany with observations of other equipments, would help us more deeply explore the low-frequency astroprocesses.