The present disclosure is provides a method, an apparatus and a terminal device for constructing parts together. The method includes: determining an assembly progress of an object constructed of parts; determining a currently required part according to the assembly progress; identifying, in a part photo of the object, the currently required part using a pre-trained first neural network model and marking the one or more identified currently required parts in the part photo; and displaying, via a display, a 3D demonstration animation with the marked part photo as a background image. The method is based on the existing 3D demonstration animation, which identifies the currently required part in the captured part photo and marks the identified currently required part in the part photo, and then displays the marked part photo as the background image of the 3D demonstration animation, thereby adding a prompt for the real part.