The invention discloses a wellhead assembly bottom connecting device for shortening surface-drilling working time, comprising a standard base assembly, a surface casing spindle hanger, an adapter joint and a wellhead assembly. A sealing unit with the outside diameter equal to the outer diameter of a surface casing is arranged at the upper end of the surface casing spindle hanger, extends to a lower inner cavity of the wellhead assembly, and is hermetically connected with the inner wall of the lower cavity of the wellhead assembly through a sealing device; a connecting unit is arranged below the surface casing spindle hanger and is in threaded connection with the adapter joint; a locking bolt connected with the upper end of the surface casing spindle hanger is arranged at the lower end of the adapter joint; a plurality of connecting bolts are arranged at the bottom of the wellhead assembly, and the adapter joint is connected with the wellhead assembly detachably through the connecting bolts. The wellhead assembly bottom connecting device has the advantages of saving time for cleaning the sealing surface, simplifying assembling of the wellhead device, shortening time for drilling andwell completion work and improving field operation efficiency.