The invention relates to a method for preparing bismuth trioxide (Bi2O3) nanotubes by a microwave-assisted liquid-phase process and application of the bismuth trioxide (Bi2O3) nanotubes in photodegradating tetracycline-containing wastewater. The preparation method comprises the following steps: magnetically stirring proper amounts of Bi(NO3)3.5H2O and Na2SO4 to form a mixed solution, carrying out ultrasonic dispersion on NaOH and deionized water to form a sodium hydroxide transparent solution, and dropwisely adding the sodium hydroxide transparent solution into the Bi(NO3)3.5H2O/Na2SO4 mixed solution; and continuously heating in a reactor with the microwave power of 800W to obtain the product, centrifuging, washing and drying to obtain the bismuth trioxide (Bi2O3) nanotubes with uniform appearance. The invention has the advantages of simple technique and reproducibility; and the raw materials are cheap and accessible inorganic compounds, are low in cost, do not need pretreatment of high temperature and calcining, and have the advantages of low synthesis temperature and short reaction time, thereby lowering the energy consumption and reaction cost and facilitating the mass production. Since the bismuth trioxide (Bi2O3) has strong photocatalytic degradation capacity for tetracycline, the invention is feasible in practical application.