A brainwave signal collecting device includes a main part and an elastic sleeve having a first opening. The main part is installed on the elastic sleeve, and the elastic sleeve can be positioned by suction on a user's head through the first opening after being pressed. The main part is in contact with the head to collect brainwave signals. The brainwave signal collecting device has an elastic sleeve serving as a flexible piece. When the brainwave signal collecting device is worn, the elastic sleeve can be pressed to partly exhaust the air therein so as to be positioned by suction on the head by the first opening of the elastic sleeve, which can improve the comfort of the head in contact with the elastic sleeve; and the position and angle at which the main part contacts the head can be adjusted through the deformation of the elastic sleeve.