Abstract: Ambient air pollution has become a widespread global public health problem. As one of the main components of ambient air pollution, fine particulate matter (PM_2.5), with its small diameter and large surface area, can carry a variety of toxic substances and enter the blood circulation directly through the blood-air barrier, damaging various tissues and organs of human body. Studies have shown that PM_2.5 exposure during pregnancy can disrupt the mother's and child's thyroid function. Since the fetal thyroid gland does not begin to develop until around the sixth week of pregnancy, the fetal thyroid hormone is almost entirely dependent on the mother during early stages of pregnancy, and maternal thyroid hormone level play a crucial role in the growth and development of fetus. When a mother is exposed to PM_2.5 during pregnancy, placenta, the "bridge" between mother and fetus, is also affected to some extent, including changes in placental iodine uptake and oxidative stress, inflammation, and DNA methylation in placental tissue. Exposure to PM_2.5 during pregnancy also alters maternal thyroid hormone level and normal placental function, which can have a detrimental effect on pregnancy outcomes, such as preterm birth, low birth weight, and neurological abnormalities. This paper reviewed the effects of PM_2.5 exposure during different trimesters on maternal and infant thyroid function, placental function, and pregnancy outcomes, aiming to provide more accurate protection of maternal and fetal health.