This paper systematically reviews the technical problems of drilling and completion fluid in deep-earth, deep-sea, unconventional, and other complex oil and gas drilling. According to the key scientific problems and their research progress, the future development direction of drilling and completion fluid technologies in deep-earth, deep-sea, unconventional, and other complex oil and gas drilling is proposed. In view of high temperature, high pressure, high salt, reservoir damage, poor rheological properties, hydrate blockage, and wellbore instability encountered in complex oil and gas drilling, global scholars have developed high-temperature and environmental protection drilling and completion fluid technology, high-temperature, high-salt, and high-density drilling and completion fluid technology, and liquid casing drilling and completion fluid technology. However, as drilled formation gets increasingly complex, there are still problems such as insufficient high temperature resistance of drilling and completion fluid materials, poor environmental protection performance, and serious lost circulation and reservoir damage. In order to meet the performance requirements of drilling and completion fluid in complex oil and gas drilling, it is necessary to further study the mechanism of drilling and completion fluid treatment agents, establish a safe and efficient multi-functional integrated control method for drilling and completion fluid, and build intelligent drilling and completion fluid theory and technology, so as to provide technical support for developing complex oil and gas resources in China.