In the field of nonlinear optics and physical quantity detection, the use of the second harmonic wave (SHW) generated in ferroelectric crystals is proposed to realize multi-physical quantity detection with the Janus property. In view of the single physical quantity detected by the current research and the single application scenario, this paper proposes a multi-functional and novel nonlinear Janus metastructure (NJMS), which exploits the SHW to achieve highly sensitive multi-physical quantity detection in the terahertz frequency range and shows Janus properties in both the forward and backward directions of the system. The NJMS is realized to detect refractive indices, thicknesses, and angles with different modes in the forward and backward directions. The proposed NJMS broadens the application scenario of the SHW and provides a novel idea for the research of multi-physical quantity detection devices with the Janus property.