Knock behavior in diesel-dual-fuel (DDF) engine is more complex, more severe, and different than those of traditional engines. We investigate a type of diesel-dual-fuel engines, where CNG is multipoint-injected at the intake ports as main fuel and diesel is directly injected in smaller amount, mainly for ignition purpose, resulting in lower fuel cost. Because of the mixed behaviors between the spark ignited and compression ignited engines, a more sophisticated control system is needed to properly control knock in the DDF engine. In this paper, a novel control system based on fuzzy logic is presented to regulate knock intensity at an appropriate level. The control system comprises a fuzzy controller and a fuzzy decision maker. The fuzzy controller controls several pertaining actuators using rule-base from human experience, while the fuzzy decision maker adapts the magnitude of each actuator action to various operating points. From an engine test bed data, the in-cylinder pressure was compared to the knock sensor signal to select a frequency range to consider. An existing frequency-domain knock intensity detection algorithm was applied to detect the knock intensity in real time. Then the control system was designed to adjust several parameters, including CNG and diesel amount, diesel injection timing, rail pressure, and throttle and EGR set points, for the engine to operate near the knock intensity limit. A switching algorithm was added to prioritize the actuators. The control system was trialed with an engine test bed, programmed to run an NEDC test. The proposed algorithm has shown excellent result in regulating knock. The DDF engine was able to operate closer to the optimal points, resulting in more CNG use at most operating points.