TianQin is a proposed space-borne gravitational wave detector designed to detect gravitational waves in the millihertz (mHz) frequency range. The experiment relies on a constellation of three drag-free spacecraft orbiting the Earth, using inter-spacecraft laser interferometry to monitor the distances between test masses. The primary goal is to detect a signal from a single source of gravitational waves within a few months of observing time. The reference source chosen for the experiment is RX J0806.3+1527 (J0806), an ultra-compact galactic binary known to emit periodic gravitational waves at a frequency of 321.5 seconds. The spacecraft will be placed in geocentric orbits with a semi-major axis of approximately 105 km, forming a nearly equilateral triangle. This configuration allows for the use of more readily available spacecraft technologies, reducing the overall mission cost to an estimated range of USD 550-800 million. The sensitivity goal of the experiment is to achieve a peak positional sensitivity of 1 pm/Hz^1/2 and a residual acceleration sensitivity of 10^-15 m s^-2/Hz^1/2. The paper discusses the preliminary mission concept, including the reference source, spacecraft orbit, sensitivity goal, and error budget for key components such as the laser interferometer and disturbance reduction system. The technology status of the laser interferometer and disturbance reduction system is also reviewed, highlighting the progress made in developing the necessary technologies. The authors expect TianQin to be launched in the second half of the next decade.TianQin is a proposed space-borne gravitational wave detector designed to detect gravitational waves in the millihertz (mHz) frequency range. The experiment relies on a constellation of three drag-free spacecraft orbiting the Earth, using inter-spacecraft laser interferometry to monitor the distances between test masses. The primary goal is to detect a signal from a single source of gravitational waves within a few months of observing time. The reference source chosen for the experiment is RX J0806.3+1527 (J0806), an ultra-compact galactic binary known to emit periodic gravitational waves at a frequency of 321.5 seconds. The spacecraft will be placed in geocentric orbits with a semi-major axis of approximately 105 km, forming a nearly equilateral triangle. This configuration allows for the use of more readily available spacecraft technologies, reducing the overall mission cost to an estimated range of USD 550-800 million. The sensitivity goal of the experiment is to achieve a peak positional sensitivity of 1 pm/Hz^1/2 and a residual acceleration sensitivity of 10^-15 m s^-2/Hz^1/2. The paper discusses the preliminary mission concept, including the reference source, spacecraft orbit, sensitivity goal, and error budget for key components such as the laser interferometer and disturbance reduction system. The technology status of the laser interferometer and disturbance reduction system is also reviewed, highlighting the progress made in developing the necessary technologies. The authors expect TianQin to be launched in the second half of the next decade.