Abstract: A set of reference moisture value of the plurality of sample skin is measured with conventional skin moisture measuring device. Then a set of standard calibration equation is provided from calculating a set of measured reflectance spectra of calibration and the set of reference moisture value by using the multivariate regression, and it is stored at a memory. The next steps are to measure skin moisture of test skin with reference to the set of standard calibration equation stored at the memory. By radiating near infrared on the test skin and sensing a set of reflectance spectra with the portable skin moisture measuring apparatus by using near infrared reflectance spectroscopy system, the skin moisture is measured at a repeatable and stable way, regardless of the variation of temperature or humidity of the external circumstance.