A conversion apparatus for catalytic cracking a hydrocarbon feed to light hydrocarbon comprises at least one riser reactor, a dense bed reactor, a disengager, and a stripper. A dense bed reactor which is separated from disengage, is employed to enforce further cracking hydrocarbon to light olefins, with low methane yield. Moreover, the spent catalysts discharged from the outlet of the dense bed reactor can be introduced into the stripper via a specific catalyst transporting channel, to maintain catalyst concentration in the dense bed reactor that can be advantageous to deeper cracking of the intermediate products to produce more light olefins, particularly propylene.