A fluid injecting system (100) comprising an injector (A) having an injector body (10), a plunger shaft (30), a plunger assembly (20) having an inner plunger barrel (21) slidable within an outer plunger barrel (22), a fluid-cartridge (B) having a fluid (53) and a hypodermic needle (72) for injecting the fluid (53) at an injectable site. The fluid-cartridge (B) is configured to releasably engage with the injector (A). A vacuum (V) is created between the outer plunger barrel (22) and the inner plunger barrel (21) upon formation of a united plunger barrel (40). Also, forward movement of the united plunger barrel (40) transfers the fluid (53) from the fluid-cartridge (B) into the injectable site. The hypodermic needle (72) retracts within empty fluid-cartridge (B) due to release of the vacuum (V) and the injector (A) is disengaged from the fluid-cartridge (B) for reuse. Further, a fluid injecting method (300) is provided.