The utility model provides a flat-plate solar collector which comprises a shell, a transparent cover plate, a heat
insulation layer, a main flow pipeline, a
branch flow pipeline, a heat absorption plate and a first connecting
assembly, a heat collection cavity is formed in the shell, the transparent cover plate is fixed to an opening in the upper portion of the shell, and the heat
insulation layer covers the inner surface of the shell. The front ends and the rear ends of the
branch pipelines are fixed to the two main flow pipelines respectively and communicate with the main flow pipelines, the heat absorbing plates are fixed to the left sides and the right sides of the
branch pipelines, the first connecting
assembly comprises a first connecting
pipe, an inner connecting sleeve and an outer connecting sleeve, threads are arranged outside the first connecting
pipe, and the first connecting
pipe transversely penetrates through the shell and is in threaded connection with the shell. Threads are arranged in the inner connecting sleeve, the left end and the right end of the inner connecting sleeve are in threaded connection with the main flow pipeline and the first connecting pipe respectively, threads are arranged in the outer connecting sleeve, the outer connecting sleeve corresponds to the outside of the shell, and one end of the outer connecting sleeve is in threaded connection with the first connecting pipe.