Second harmonic wave (SHW) generation is a nonlinear optical phenomenon, wherein electromagnetic radiation undergoes a process of doubling its frequency. A multitask Janus metastructure (MJMS) based on liquid crystal (LC) tunable defects is introduced in this article, enabling temperature detection and biosensing. Due to the asymmetric arrangement of dielectrics, MJMS exhibits Janus characteristics in both forward and backward directions. Significant enhancement in the output efficiency (OE) of SHW can be achieved through the adjustment of LC defects and structural design. When the cavity under test is filled with air, high-precision temperature sensing can be achieved by detecting the shift of the SHW’s peak (SHWP), with a detection range of $15~^{\circ }$ C– $40~^{\circ }$ C. The highest sensitivity (S) can reach 0.00164 THz/°C. When the cavity under test is filled with analytes, different types of waterborne bacteria can be identified on the forward scale by the amplitude shift of SHWP, with a measurement range of 1.365–1.425 and $S = 0.9826$ /RIU. On the backward scale, MJMS can detect glucose solution with concentrations ranging from 0 to 400 g/L with a detection range of 1.33–1.38 and $S = 3.402$ /RIU. This MJMS has the potential to drive breakthroughs in electromagnetic detector design, opening up new avenues for exploring SHW applications and LC research.