Abstract: An active target time projection chamber (AT-TPC) was designed and its performance was tested. The chamber has an active tracking volume of 10 cm×10 cm×14 cm, which is equipped with two layers of thick gas electron multiplier (THGEM) and connected to 64 readout strips. A guard ring was also integrated above the GEM foils in order to adjust the electric field in the gas vessel, which allowed to improve the overall performance of the system. At a guard ring HV of -950 V, the AT-TPC achieved a tracking position resolution better than 0.2 mm, an angular resolution below 0.6° and a time resolution better than 20 ns, obtained from α particles test. By using the gas mixture 96%He+4%CO₂ as the sensitive working gas as well as the target material, the α+α elastic scattering events can be clearly imaged.