Abstract: Aiming at the requirements of the mode and operating temperature of vertical-cavity surface-emitting laser (VCSEL) used as the laser source system of the atomic clock (Cesium) chip, the 894.6 nm oxide-confined fundamental transverse mode VCSEL that could operate at high temperature was reported. By reducing the diameter of the oxide aperture of the VCSEL to 3 μm, the higher order transverse modes could be suppressed, which guaranteed the fundamental transverse mode and low threshold current of the VCSEL. Through the structural design that the cavity mode and the material gain was detuned by 12 nm at room temperature, the emission wavelength of the device could match with the atomic energy level and the operating mode was stable at a high temperature of 50-65 ℃. The obtained VCSEL shows a center wavelength of 894.6 nm, a side mode suppression ratio (SMSR) larger than 35 dB, a fundamental transverse mode power of 0.75 mW and a far-field divergence angle of 11.4° when the operating temperature is 55 ℃ and the injection current is 1.8 mA. At the temperature of 65 ℃, the SMSR is larger than 25 dB and transverse mode power is larger than 0.1 mW. The development of the high temperature fundamental transverse mode VCSEL has great potential in chip atomic clocks.