The invention relates to a tightening prevention safe water conservation disconnecting valve with an adjustable length of a pipe connecting union. The disconnecting valve comprises a valve body and is characterized in that the valve body is provided with a valve-stem nut; an internal screw hole of the valve-stem nut is connected with a valve handle by an internal screw thread; an internal hole ofthe valve handle is internally and movably connected with a pressure spring and a valve clack which is arranged on a valve seat arranged in the valve body; the lower end of the valve seat at one sideof a water outlet end of the valve body is sealed; the upper end of the valve seat at one side of a water inlet end of the valve body is sealed; the sum of the length of the screw thread on the valvehandle and the thickness of the valve clack is less than equal to the distance from the internal screw hole of the valve-stem nut to the valve seat; the water inlet end of the valve body is connectedwith an adjusting screw nut of a junction block by the screw thread and is internally provided with a clamping ring of the junction block and the junction block which is movably connected with a connector nut. The disconnecting valve both can effectively avoid the damage of the valve clack caused by excessive screwing a hand wheel and regulate the length of the pipe connecting union.