The invention relates to a horizontal rotary cylinder type biogas slurry in-situ quality-improving device and method. The horizontal rotary cylinder type biogas slurry in-situ quality-improving device has a structure mainly comprising an outer cylinder, an inner cylinder and a motor; the inner cylinder is provided with an inner cylinder cover, the inner cylinder cover is provided with a feed inlet, and an inner cylinder wall is provided with equally spaced sieve holes; the outer cylinder is provided with an outer cylinder cover, an inner wall of the outer cylinder is provided with a scraping plate, the lower side of the cylinder wall is provided with a biogas slurry outlet pipe and a support, and the upper side of the cylinder wall is provided with a gas guide port; the motor is connected with the inner cylinder by a driving mechanism; the rotating inner cylinder can play a role in stirring materials, also can centrifuge and filter biogas slurry through the sieve holes in the wall. In-situ purification of the biogas slurry can be achieved, liquid discharge and slag cleaning are convenient, inconvenience of restriction of transportation distance and land occupation area brought by traditional biogas slurry displacement filtration is avoided, the safety, high efficiency, flexibility and convenience of biogas slurry agricultural utilization are achieved, and great significance is provided for promotion and popularization of biogas slurry agricultural utilization.