A 1550 nm linearly tunable external cavity semiconductor laser (ECSL) based on single cavity all-dielectric thin-film Fabry-Perot filter is proposed and realized here. Its internal optical components as well as their operation mechanisms are introduced one by one first, and then its longitudinal mode output characteristic is analyzed theoretically. At last, the experiment platform for the output characteristics measurement of the tunable ECSL is set up; under different experimental conditions and with optical spectrum analyzer, scanning Fabry-Perot interferometer versus laser power meter, accurate and real-time measurements for the output wavelength, output line-width and output power of the tunable ECSL during its tuning process are executed. The optimal experimental condition from the measured datum is summed up, and the tunable ECSL relevant parameters under this condition is obtained. The tunable ECSL will have a linear mode-hop-free wavelength tuning region of 1547.203~1552.426 nm, a stable output optical power in the range of 40~50 μW, and a stable output longitudinal mode distribution of a single longitudinal mode with a line-width in the range of 100~150 MHz. This tunable ECDL could be used in the application of environmental monitoring, atomic and molecular laser spectroscopy research, precise measurements, and so on.