This paper reports a new design of tunable thermal gradient index (GRIN) lens using the thermal diffusion between one liquid [1], and its application in dynamic trap of single living HEK 293 cell. This system consists of the GRIN lens part and the cell trapping part. The former includes a trapezoid and a rhombus region to form a GRIN lens. The optical properties can be modulated by flow rates and liquid temperature. The latter is isolated to the GRIN lens by a very thin PDMS wall for dynamic cell trap. The cells flowing in this region will be trapped and analyzed by the synergy of optical forces and drag forces. Unlike other counterparts using concentration diffusion [2] and thermal diffusion [3], this thermal GRIN lens is formed by thermal diffusion between only one liquid, and the RI along radial direction fits to a parabolic variation, and its enhanced performance shows more flexibility of controlling single living cell as compared to the solid optical tweezers.