Theophylline, an organic compound with the formula C7H8N4O2, reduces smooth muscle tone and dilates the respiratory tract. Can promote the release of endogenous gonadin, airway smooth muscle relaxation; Inhibiting the release of calcium ions from the smooth muscle endoplasmic reticulum and reducing the intracellular calcium concentration resulted in respiratory tract dilation. Theophylline has stronger relaxation effect on smooth muscle, but not as strong as β-receptor agonist