The invention discloses an activated rosin flux for titanium alloy gas shielded welding under a thickness of 6 mm. The 6 mm is the thickness of the weldment to be welded. The gas shielded welding comprises an argon arc welding, a laser welding and submerged arc welding. The activated rosin flux comprises, by weight, 25-30 of MgF2, 16-22 of CaF2 and 8-12 of SrF2. The margin is NaF. The preparation method of the activated rosin flux comprises the following steps: components of the activated rosin flux are physically mixed in proportion and are melted under a high temperature of 1450 DEG C. The melted chemical compounds are disintegrated mechanically to a particle size of less than 0.063 mm to get the activated rosin flux. The activated rosin flux has the advantages of compressing a welding arc, enhancing the penetrating power of an arc, enlarging welding penetration, improving features of an arc technology and the morphology of a welding line, decreasing weld heat input, refining the welding line and heat affected zone grains and eliminating weld porosities.