Ion signals play important roles in different kinds of biomedical processes. Traditionally, the detection methods usually require a relatively large sample size and expensive equipment to perform the ion concentration measurements. Here we proposed to use organic electrochemical transistors (OECTs) to measure the electrochemical signals from the ions. Comparing with the traditional methods, OECTs are conformal and flexible, more importantly, able to transfer to different objects to perform directly in-situ sensing. The OECT can also perform signal amplification function via transduce ion concentration signals to electrical signals. Here we develop the OECTs based on the structure of gold source and drain contacts and ion-permeable conducting polymer Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)(PEDOT:PSS) as channel. From the experimental results of different metal ions such as Ca2+ and Na+, the ion sensitivity can be up to 2.5 times of the theoretical Nernst limit at a few hundred millivolts low-voltage operation. We also develop the ion-selective OECTs to detect the body fluid and we will fabricate this device on flexible substrate for future application in real-time ion signals detection. The unique performance of our devices provide new opportunities in the field of implant biosensors for biomedical diagnostics.