The invention discloses a fault online diagnosis method of uninsulated track circuit tuning zone, which relates to the online diagnosis of a track circuit. The method comprises the steps of obtaining cab signal inductive voltage amplitude envelope, shunt point positions, train speed and track circuit signal carrier frequency information, pretreating the data to obtain the length of the track circuit, the quantity and position of compensation capacitors and cab signal inductive voltage normalized amplitude envelope, performing segmented fitting on the cab signal inductive voltage normalized amplitude envelope and establishing corresponding feature vectors, establishing a neural network, training the neural network by utilizing the feature vectors and performing fault diagnosis on the uninsulated track circuit tuning zone equipment, and sending diagnosis results of the tuning zone equipment to a corresponding ground monitoring client-side through a cab signal remote monitoring system. The method has the characteristics of being complete in functions, high in detection accuracy, high in applicability, simple in method, good in real-timeness and the like, no any hardware equipment is added in the method based on the existing cab signal system, and the cost is low.