According to the strain and temperature sensitivities of long period gratings(LPFG), a fiber optical sensor with higher sensitivity was developed. Two long-period fiber gratings with different cladding parameters and periods were in series. With detailed numerical simulation, it illustrated that this new sensor is more sensitive to temperature and strain which could be measured together. Further analysis for the factors affecting the sensitivity of the sensor indicated that the higher mode of coupling times is more sensitive than the lower one, which helps to improve the sensor’s sensitivity.