We report a comprehensive high-pressure study on the monoclinic TlFeSe2 single crystal,which is an antiferromagnetic insulator with quasi-one-dimensional crystal structure at ambient pressure.It is found that TlFeSe2 undergoes a pressure-induced structural transformation from the monoclinic phase to an orthorhombic structure above Pc ≈ 13 GPa,accompanied with a large volume collapse of △V/Vo =8.3％.In the low-pressure monoclinic phase,the insulating state is easily metallized at pressures above 2 GPa;while possible superconductivity with Tconset ～ 2 K is found to emerge above 30 GPa in the high-pressure phase.Such a great tunability of TlFeSe2 under pressure indicates that the teary AFeSe2 system (A =Tl,K,Cs,Rb) should be taken as an important platform for explorations of interesting phenomena such as insulator-metal transition,dimensionality crossover,and superconductivity.