Gyrotron can achieve high peak power and high average power output in millimeter wave and terahertz band， which has important application requirements. Based on the beam wave interaction theory of gyrotron， a 140GHz gyrotron is designed for test and aging process of the relevant elements of electron gyrotron heating in magnetic confinement fusion. The goal is to achieve a maximum output power of not less than 50KW which can operate in pulse and continuous mode， and a certain range of frequency and power adjustment to meet the needs of the test. According to the design results， the gyrotron is developed and tested. A maximum output power of 56 kW is achieved corresponding to the cathode voltage of -37.2 kV， the modulation anode voltage of -12.19 kV， the second anode voltage of +11 kV， the beam current of 3.4 A and the magnetic field of 5.3 T. The power can be adjusted by changing beam current and magnetic field. In the same way， the frequency can be adjusted within 80 MHz. The theoretical calculation results of starting current， power and frequency are compared with the experimental results， which are in good agreement. In the preliminary experiment， overheating of the window was observed with an output power of 52 kW for 30s’ operation， so the continuous operation experiment was carried out with the power reduced to 20.3 kW， under this power level， the continuous operation state of the gyrotron is stable， which can be used for the subsequent work for test and aging of fusion elements.