The invention relates to a windscreen wiper with adjustable length. The windscreen wiper comprises a rocker arm and a windscreen wiper blade assembly, wherein the windscreen wiper blade assembly comprises a main frame, first auxiliary supports, second auxiliary supports and a rubber strip; two ends of the main frame are connected with the first auxiliary supports respectively; one end of each first auxiliary support is connected with the corresponding second auxiliary support, and the other end of each first auxiliary support is connected with a sliding chute; the rubber strip comprises a fixed section and movable sections arranged at two ends of the fixed section, two ends of the fixed section are connected with one ends of the two sliding chutes, and the other ends of the two sliding chutes are glidingly connected with clamping grooves for mounting the movable sections respectively; each clamping groove is provided with a connecting seat which is connected with the other end of the corresponding first auxiliary support through a compressed spring; the windscreen wiper further comprises a tensioning device for keeping one end surface of each of the two movable sections of the rubber strip to be attached to each of two end surfaces of the fixed section. According to the windscreen wiper with adjustable length, lengths of windscreen wiper blades can be increased during working, so that the larger wiping area can be obtained while the service life of a motor cannot be affected.