To meet the application requirements of the weak energy harvesting system for low supply voltage and low power consumption, a low power and small area relaxation oscillator with a low temperature coefficient was designed. The structure of the self-cascode composite transistor was used to replace the large resistance in the traditional beta-multiplier-based current references and the large resistance in the oscillator core circuit that generates voltage reference at the comparator input, so as to reduce the circuit area and improve the integration while achieving low power consumption. Based on a 0.18 μm CMOS IC process, the simulation results show that the oscillator can work normally at a supply voltage of 0.8-1.2 V, with a frequency of 2.2 kHz and a minimum power consumption of 30 nW, achieving a temperature coefficient (TC) as low as 1.03×10-4/℃, and the die area was reduced by at least 70% compared with previous research.