The invention relates to a multi-slug composite pressure decreasing and injection increasing agent for an ultra-low permeability oil field and belongs to the technical field of chemical pressure decreasing and injection increasing of the ultra-low permeability oil field. According to the agent, active retarded acid, a complex antiscaling agent, a shrinking agent and an antisludging agent are integrated, and a multi-slug and staging injection mode is adopted, so that the plugging of the formation of a water injection well of the ultra-low permeability oil field is effectively eliminated, the injection allocation of an under injection well is realized, and the water injection pressure of the under injection well is reduced, the energy of the formation is supplemented, and the recovery efficiency of crude oil is improved. The problem of low acidification effect and short effective period of measures, which are caused by shortened penetrating distance, the problem that the effects of an active agent are reduced because the active agent cannot reach a far well zone, and the problems of reduced permeability, shortened effective period, ineffectiveness or increased water injection pressure, which are caused by incompatibility of formation fluid and new generated precipitates in the process of applying the conventional single pressure decreasing and injection increasing technology to the ultra-low permeability oil field at site can be solved. The results of a site test prove that the effective rate of the measures can reach 91.3 percent, and the pressure of a single well can be decreased by 1.5MPa averagely.