The invention discloses a relaxation oscillator comprising a first capacitance, a plurality of controlled switches, a second capacitance which is connected with a plurality of controlled switches to form a parallel circuit between the first capacitance and the second capacitance through the on-off of a plurality of controlled switches, a current supply which is connected with the parallel circuitto provide charging current, a comparator which is connected with one end of the parallel circuit to compare the voltage of the parallel circuit with a preset reference voltage so as to generate an impulse signal from the comparison result and output the impulse signal, and a trigger module which is connected with the output end of the comparator to generate control signals of a plurality of controlled switches according to the impulse signal so as to control the on-off of a plurality of controlled switches. The invention has the beneficial effects of largely reducing parts of the relaxation oscillator, reducing noise and power consumption and ensuring sufficient capacitance discharge time.