The present invention relates to the field of medical instrument technology, more particularly, to an intravascular ion sensor and the preparation method thereof. The sensor comprises a conductive substrate and the surface of the conductive substrate is provided with a layer of precious metal. The surface of the precious metal layer is provided with an ion selection membrane layer, and the surface of the ion selection membrane layer is provided with a high-molecular organic material layer to have the ion selection membrane fixed. The surface of the high-molecular organic material layer is grafted with an anticoagulation material layer. According to the invention, through an ion selection membrane layer fixed by the high-molecular organic material layer evenly sprayed on the surface of the selection membrane layer and the anticoagulation material layer grafted on the surface of the high-molecular organic material layer, the sensor is provided with certain anticoagulation function, enabling it to have an anticoagulation performance after working for 10 to 20 hours in the blood current of a blood tube. And the sensor can be used directly in human blood vessels to detect the concentration of electrolyte without the occurrence of thrombosis occlusion in the effective time and provide a convenient and advanced detection solution to a citrate partial anticoagulation machine and other instruments for the online monitoring of blood components.