Biochar is an excellent soil amendment, but its fertilizer effect on the soil is often overlooked. In this study, a series of experiments were conducted to investigate the form changes of P (phosphorus) and K (potassium) in rice straw biochar at different pyrolysis temperatures and explore its fertilizer effect characteristics. The results showed that in the rice straw, P was mainly in the form of OP (organophosphorus) and NaOH-P (NaOH extractable phosphorus), and K as WSK (water-soluble potassium). For biochars, the various forms of P at 100–200 °C and K at 100–300 °C were basically unchanged; OP in rice straw biochar decreased gradually from 200 °C to 800 °C; at 200–1000 °C, NaOH-P in biochar gradually decreased, with 9.1 % of P (wt%) at 1000 °C; HCl-P (HCl extractable phosphorus) with more bioavailable, which was mainly converted from NaOH-P and OP, increased gradually from 6.8 % to 50.9 % for P (wt%) at 200–1000 °C. At 100–500 °C, the change of TK (total potassium) was slight; from 500 °C to 900 °C, WSK decreased from 84.7 % to 23.2 % for K (wt%); at 700–1100 °C, the ASK (acid-soluble K) was relatively high, with K (wt%) of 10.6–11.6 %. The volatilization was mainly at 200–500 °C for P and 500–900 °C for K, which volatilization rates were 5.6–30.3 % and 6.3–58.9 %, respectively. Moreover, HCl-P increased from 0.167 mg/g for the raw straw to 4.554 mg/g for the 1000 °C biochar and TK from 3.632 mg/g for the rice straw to 20.91 mg/g for the 600 °C biochar. It can be concluded that the biochars at 400–700 °C were suitable for use as a slow-release P fertilizer and K fertilizer. This work provides a strong basis for using a rice straw biochar at different temperatures as a P and K fertilizer.Capsule: By pyrolysis, the rice straw biochar contains more HCl-P and TK, and can be used as slow-release P fertilizer and K fertilizer.