In the present study, an attempt was made to upgrade the characteristics of rice husk biochar through a combination of two strategies: (i) microwave‐assisted phosphoric acid pretreatment (MAPP) of rice husk followed by (ii) co‐pyrolysis of pretreated rice husk with low‐density polyethylene (LDPE). The analysis of pretreated rice husk through biochemical composition, proximate analysis, ultimate analysis, thermogravimetric analysis (TGA), and X‐ray diffraction (XRD) showed that MAPP significantly improved the properties of rice husk. The pretreated rice husk had higher fixed carbon and volatile matter as compared with untreated rice husk (19.65% v/s 13.35% and 66.24% v/s 61.93%) and its ash content was lower than untreated rice husk (9.75% v/s 16.85%). The TGA analysis showed that MAPP improved the thermal stability of rice husk. At 719.65 °C, the residual masses of rice husk and pretreated rice husks were 13.76% and 23.75% respectively. An analysis of properties of biochars revealed that, biochar produced through combined mode had the highest carbon content (86.14%), the least ash content (1.29%), the least H/C and O/C values (0.29 and 0.09), and the highest Brunauer Emmett and Teller (BET) surface area (35.022 m2/g). Raman spectroscopy and XRD results showed that the biochar had a more orderly structure and less structural defects as compared to biochar produced from rice husk alone. © 2022 Society of Chemical Industry and John Wiley & Sons, Ltd.