A frequency locking method for a phase-locked loop comprises the following steps: S1, a frequency control module controls a numerically controlled oscillator to obtain an maximum output frequency and a minimum output frequency; S2, obtain a minimum frequency ratio and a maximum frequency ratio by means of a time-to-digital converter and the frequency control module; S3, calculate a first frequency control word and a first frequency ratio according to the minimum frequency ratio and the maximum frequency ratio; S4, the frequency control module uses the Newton's iterative method to recalculate a new frequency control word; S5, obtain a new frequency ratio according to the new frequency control word; S6, if the new frequency ratio is within an error range, end iteration and stably output the new frequency control word, and otherwise, jump to step S4.