Polydimethylsiloxane (PDMS) is a good choice for the substrate and encapsulation of clinical flexible electronics, since it possesses some distinguished characteristics such as high elasticity, excellent optical characteristic, good biocompatibility, and stability. In the present study, the emulsion polymerization technique was used once more to fabricate porous PDMS, which is expected to assure the sweat penetration through the flexible electronics, and therefore to reduce the irritation to the skin due to the flexible electronics. To assess the mechanical performance of flexible electronics with moisture, the saturated moisture concentration, coefficient of moisture expansion, and elastic modulus of porous PDMS for different relative wetness fraction were measured in experiment. Meanwhile, an asymptotic homogenization method (AHM) was adopted to predict these parameters theoretically. Results indicate that the saturated moisture concentration is linear to the porosity, while the coefficient of moisture expansion is independent of the porosity, both of which are well verified by the experimental data. The fitted formula on the elastic modulus for different porosities suggested in our previous study was developed to take account of the relative wetness fraction based on the experimental data. These three parameters were finally applied in calculating the stretchability of a flexible electronic with serpentine interconnects in moist environment. Numerical stimulation reveals that the stretchability increases with the porosity and relative wetness fraction of the substrate and encapsulation. The present work is hoped to pave the way for flexible electronics in clinical applications.