Abstract: The LTC Levelogger Junior is one of the most small and convenience instruments in monitoring groundwater level, temperature and electrical conductivity. It is widely used in monitoring groundwater dynamics and seawater intrusion. The system error of water pressure and salinity will increase over time and is always ignored. In this paper, a simple and effective method is proposed to test and calibrate the system error of water pressure and salinity by using the 52 LTC Levelogger Juniors in laboratory. Statistical analyses indicate that almost 71.2% and 90.4% of the LTC Levelogger Juniors exceed the system accuracy when measuring water pressure and salinity, respectively. The system error is calibrated in this paper. It turns out to be a good linear relationship between the measured and actual values of pressure and salinity. Each LTC Levelogger Junior has different calibrated parameters. The system error of water pressure and salinity ranges from -2.08 cm to 5.21cm and from -10.77 g/L to 2.03 g/L, respectively, before calibration, but ranges from -1 cm to 1 cm and from -1 g/L to 1 g/L, respectively, after calibration. The results show that the LTC Levelogger after should be test and calibrated before applied in the field.