The invention provides a valve core, a delay brake valve and a plunger pump. The valve core comprises a core shell and a core body arranged in the core shell, wherein an oil drainage assembly is arranged on the core body; the core shell is provided with an inlet and an outlet which are oppositely formed; the core body comprises a first end and a second end which are oppositely arranged; the inlet,the outlet, the first end and the second end are arranged in one-to-one correspondence; the oil drainage assembly comprises oil drainage grooves which are formed in the circumferential surface of thecore body; and oil is throttled through the oil drainage grooves.