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Soil acidification, fertility decline, and continuous cropping obstacles are detrimental to Chrysanthemum morifolium Ramat. ‘Hangbaiju’ growth. Biochar has been reported as a good soil conditioner. However, the mechanism by which rice-straw biochar regulates soil chemical and biological properties and adjusts Hangbaiju growth needs to be elucidated. A field experiment was conducted in Tongxiang City, China, where Hangbaiju originated, to determine the effect of rice-straw biochar on the characteristics of soil nutrient and microorganisms and on yield, quality, and nutrient use efficiencies of Hangbaiju. Significant increases were observed in total N content in soil and soil pH in all biochar treatments (5, 10, 15, and 20%, w/w), in NH₄OAC-extractable K and organic C content by the application of 10% or more rice-straw biochar, and in Olsen P content by the addition of 20% biochar compared with that of no biochar application in soil (CK). However, alkali-hydrolyzable N content was significantly reduced by 25.9 to 35.3% in all biochar treatments. Application of 20% biochar significantly increased the bacterial quantity of soil by 26.9% compared with that of CK. Applying 10% or more biochar significantly increased actinomycete quantity by 80.6 to 130% but decreased the number of fungi by 22.2 to 30.2%. Furthermore, Fusarium oxysporum in all biochar treatments was reduced by 42.4 to 54.4% compared with CK. Hangbaiju yield was greatest when 10% of biochar was added to the soil (23.9% higher than CK). However, larger amounts of biochar decreased yield. Biochar application increased the total flavonoid content in the flower of Hangbaiju compared with CK by 9.40 to 43.3%. However, biochar application decreased physiological P and K use efficiencies but caused a slight increase in physiological N use efficiency. Therefore, the application of 10% (w/w) rice-straw biochar in soil is best suited to improve soil quality and increase the yield of Hangbaiju.