This paper introduces a controlled temperature system in which the silicon controlled rectifier is controlled by a TP-801 microcomputer. A common-used PID expression is usually applied to control temperature, but it is necessary to adjust three parameters, making the operation quite inconvenient. This system adopts the measurement of a simplified expanded-critical proportion, only one parameter is necessarily adjusted. In addition, the method of digital triggering is applied so that the D/A transfer and silicon adjuster are omitted, thus decreasing the cost and increasing the reliability of operation. This system has higher precision of control and better degree of linearity of temperature increment or decrement.