The invention provides a wellhead back pressure controlling system of managed pressure drilling and a wellhead back pressure controlling method. The wellhead back pressure controlling system of the managed pressure drilling comprises a slush pump, a managed pressure automatic choke manifold, a liquid-gas separator, a slush pool, a rotary blowout preventer and a slush diversion manifold system, wherein the slush pool is respectively connected with the slush pump, the managed pressure automatic choke manifold and the liquid-gas separator. The slush diversion manifold system comprises a branch path I, a branch path II, a branch path III, a branch path IV and a branch path V, wherein the branch path I, the branch path II, the branch path III and the branch path IV are connected through a cross t3, the branch path V is connected with the branch path II through a tee t5, and the branch path V and the branch path IV are connected through a tee t8. The wellhead back pressure controlling system of the managed pressured drilling is adopted by the wellhead back pressure controlling method. The slush diversion manifold system replaces a back pressure pump system, operational cost is reduced, a site area is saved, and high-fault factors are removed.