The invention discloses a cylindrical wide-mouth bottle for detecting the water content or volume weight or decomposition degree of organic fertilizer and relates to a detection instrument of the organic fertilizer. The diameters of a bottle body and bottle opening are both 10.84 cm, the height of the bottle is 10.84 cm, the capacity of the bottle is 1L, and two protrusive rings are arranged on the bottle wall below the bottle opening. By using the wide-mouth bottle, the water content or volume weight or decomposition degree of the organic fertilizer can be detected. When the water content isdetected, the organic fertilizer is naturally air-dried and does not need to be dried by a vacuum oven because the content of volatile components in the organic fertilizer is low; when the volume weight is detected, the influence of different weights and time points on the measurement of the volume weight of an organic fertilizer sample is not obvious, and detection is directly carried out withoutadding weights; because the volume weight of the organic fertilizer sample in a sieving state is close to that in a drying state, detection is directly carried out without drying. During detection ofthe decomposition degree, whether or not the organic fertilizer is decomposed is judged directly according to the swelling degree of a sealing film. The detection method is simple and convenient to use, more reliable and accurate in result, and more beneficial to protection of the interests of farmers.