The invention relates to a diesel low-exhaust-temperature fault prediction system and a method. The diesel low-exhaust-temperature fault prediction system comprises a signal acquisition system, a modeling simulation system, and a fault diagnosis system, the signal acquisition system is used for acquiring and processing signals and transmitting the signals to a monitoring computer for storage and processing, the modeling simulation system builds a diesel combustion chamber model by employing a diesel simulation software and simulates the normal work state of a diesel, and the fault diagnosis system analyzes reasons of the fault phenomenon of low exhaust temperature, realizes the simulation of the above fault by adjusting related parameters in the modeling simulation system, establishes a fault database, compares the practical work state of the diesel with the fault database, and determines whether the diesel is in a fault state. According to the system and the method, the installation of a sensor is not needed, the fault of low exhaust temperature of the diesel can be accurately monitored in real time, the fault of low exhaust temperature of the diesel is predicted, fault diagnosis suggestions are provided, and technical support is provided for high-efficiency processing of the faults for users.