A single-phase bridgeless insulated power factor adjustment circuit includes an EMI filter module, low-frequency switching diode module, two switches and two insulated voltage transformation modules. The EMI filter module is coupled to an AC power. The low-frequency switching diode module is coupled to the EMI filter module. The two switches are coupled to the low-frequency switching diode module. The two insulated voltage transformation modules are coupled to one of the two switches. With the low-frequency switching diode module being in first ON state, one of the two switches turns on, and one of the two insulated voltage transformation modules turns on. With the low-frequency switching diode module being in second ON state, the other switch turns on, and the other insulated voltage transformation module turns on. Hence, the circuit is unlikely to fail, but features simple circuitry, incurs low costs, be compact, and achieves high conversion efficiency.