This letter presents the fabrication method of flexible singlecrystalline germanium nanomembrane （GeNM）pintrinsicn (PIN) diodes on plastic substrate and radio frequency (rf) characterization under off state of various bending strains. In order to quantitatively research the RF characteristic variations with different mechanical stress applied on the flexible PIN diode under reverse mode, accurate equivalent circuit models were set up under different bending radius. After researching the model parameters of the diode varying with the mechanical stress, it can be obtained internal resistance, parasitic inductance, p+p junction resistor and pn+junction capacitance are the main factors affecting the rf characteristics. Mechanical bending makes these parameters change monotonously, which results the rf characteristics of flexible singlecrystal germanium PIN diode getting better under the off state. It also shows great potential in the strain measurement field.