Embodiments of the invention disclose a rectifier and an uninterruptible power system (UPS). In the embodiments of the invention, diodes and resistors are connected in series to form a circuit which is regarded as a soft start circuit in the rectifier so as to precharge a bus of a rectification circuit, therefore, a soft start of the bus in the rectification circuit can be realized. In the prior art, thyristors are adopted to carry out the soft start. Compared to the prior art, software is not needed to be used to carry out complex control in the invention so that realization is simple and a failure rate is low. In addition, because costs of the diodes are lower than the costs of the thyristor, costs of the invention can be reduced.