In this study, we collected soybean inter-root soil (clay soil) from the cold region of Heilongjiang Province, China, screening for cold-tolerant phosphorus- and potassium-solubilizing bacteria by the gradient-cooling-directed screening and design of mixed bacterial agents. This study screened cold-tolerant phosphorus-solubilizing and potassium-solubilizing bacteria by gradient-cooling-directed screening and constructed mixed bacterial agents. We analyzed the strain’s phosphorus/potassium solubilizing capacity, as well as its organic acid secretion ability, to reveal its mechanism of detoxification of phosphorus and potassium. Clay soil from the cold region of Heilongjiang, China, was collected for soybean potting trials. The soybean pot experiment was used to investigate the growth-promoting effects of cold-tolerant strains applied to soybeans and to analyze the effects of changes in soil bacterial activity on soybean growth. A dominant phosphorus-solubilizing bacteria strain P-5 (Pantoea ananatis) was screened, and the amount of phosphorus-solubilizing bacteria was 34.94 mg/L with a PSI (Phosphorus Solubilization Index) of 1.80 for 7 days of incubation at 15 °C. The OD₆₀₀ value (optical density of 600 nm) on the 10th day was 0.61. The dominant potassium-solubilizing bacteria, K-1 (Rahnella aquatilis), solubilized 23.02 mg/L of potassium under the same culture conditions, with a KSI (Potassium Solubilization) of 1.51 and an OD₆₀₀ value of 0.59 on day 10. The dominant mixed bacterial agents H-5 (25% P-5 and 75% K-1), designed with the above two, detoxified phosphorus at 62.58 mg/L and potassium at 37.59 mg/L under the same culture conditions. It was shown that P-2 secretes gluconic acid and P-5 secretes lactic acid to degrade insoluble phosphorus at low temperatures and that K-1 secretes citric acid to degrade insoluble potassium. The soybean pot experiment study showed that the application of phosphorus-solubilizing bacteria (PSB), potassium-solubilizing bacteria (KSB), and mixed bacterial agents (MSB) alone could enhance the relative abundance of Pseudomonas, Rhizobium, and other bacterial genera in the soybean planting soil, compared with the control treatment without bacterial agents (BLK). Indirectly, they can enhance the accumulation of phosphorus and potassium in various organs of soybean, promote the development of soybean leaves and stems, and achieve the effect of promoting production and increasing income. This study has important theoretical value for the replacement of chemical fertilizer application by microbial agents in the cold region of China, and it has important practical and strategic significance for the promotion of income green safety in China’s main grain-producing areas.