Thermal poling of silica optical fibers can produce permanent second-order nonlinear and linear electooptic effect in fused silica fibers. In order to investigate the effects of processing parameters in thermal poling, an in situ testing system used for thermal poling process is constructed with an all polarization maintaining fiber Mach-Zehnder interferometer and is used for the in situ monitoring the thermal poling optical fibers. The effects of processing parameters in thermal poling, such as applied poling voltage, poling duration and ambient temperature, on the linear electrooptic effect created in the optical fiber are investigated. Experiments of the in situ monitoring show that the stronger the electric field in the fiber core is the higher linear electrooptic coefficient created in the fiber core could be. Considering the electrical breakdown of dielectric medium between electrodes for the structure of side polished fiber, the range of voltages of 3～4 kV could be applied during thermal poling. When around 3 kV is applied on the device with side polished fiber, the optimum poling duration is about 16 min and the optimum ambient temperature for thermal poling is about 190 ℃.