A new scheme about a semi-etched long-period fiber grating (LPFG) is proposed based on the excellent relation between cladding effective refractive index and cladding radius. A LPFG is divided into two sections with equal length. One section is eroded using HF acid. So the two sections can be regarded as two LPFGs with different resonant wavelengths. Spectral characteristics of this in-series LPFG are analyzed using dispersion equation of three-layer dielectric fiber. It is found that the distance between two resonant wavelengths increases with the decrease of the cladding radius, and the distance also increases with higher-order mode. At the same time, strain and temperature are measured synchronously, and the sensing precisions of strain and temperature are ±8.9 με and 1.4 ℃, respectively. Semi-etched long period fiber grating can solve the cross sensitive problems of different physical quantities effectively.