A method for preparing styrene and alpha-methylstyrene concurrently, the method comprising: (a) a step of performing dehydrogenation reaction of ethylbenzene and cumene in the presence of a catalyst; (b) a step of recovering heat from a reaction product by the reaction; (c) a step of separating a gas phase from at least a portion of a reaction product undergone step (b) and sending thereof to a compression part, separating condensing water including water from a liquid phase, and recovering a fraction including styrene and alpha-methylstyrene; (d) a step of compressing and cooling at least a portion of a gas phase among reaction products undergone step (c) to separate components having a low boiling point including hydrogen and carbon dioxide as a gas phase, and recycle a liquid phase including styrene, alpha-methylstyrene, ethylbenzene and cumene to step (c); (e) a step of distilling and separating at least a portion of the fraction including styrene and alpha-methylstyrene, which is recover in step (c), into a fraction including styrene and a fraction including unreacted cumene; (f) a step of recovering styrene from at least a portion of the fraction including styrene of step (e) and distilling and separating the fraction including unreacted ethylbenzene; (g) a step of recovering ethylbenzene from at least a portion of the fraction including unreacted ethylbenzene of step (f) and recycling to a reaction part, and distilling and separating the fraction including benzene and toluene; (h) a step of recovering cumene from at least a portion of the fraction including unreacted cumene of step (e) and recycling the recovered cumene to a reaction part, and distilling and separating a fraction including alpha-methylstyrene; and (i) a step of recovering alpha-methylstyrene from at least a portion of the fraction including alpha-methylstyrene of step (h), and distilling and separating a fraction including a remaining component having a high boiling point, wherein at least one of energy used during the distilling and separating in step (e) and energy used during the distilling and separating in step (h) is recovered to use for the vaporization of ethylbenzene and cumene of step (a) or as a heat source of a preparation process. (a) a step of performing dehydrogenation reaction of ethylbenzene and cumene in the presence of a catalyst; (b) a step of recovering heat from a reaction product by the reaction; (c) a step of separating a gas phase from at least a portion of a reaction product undergone step (b) and sending thereof to a compression part, separating condensing water including water from a liquid phase, and recovering a fraction including styrene and alpha-methylstyrene; (d) a step of compressing and cooling at least a portion of a gas phase among reaction products undergone step (c) to separate components having a low boiling point including hydrogen and carbon dioxide as a gas phase, and recycle a liquid phase including styrene, alpha-methylstyrene, ethylbenzene and cumene to step (c); (e) a step of distilling and separating at least a portion of the fraction including styrene and alpha-methylstyrene, which is recover in step (c), into a fraction including styrene and a fraction including unreacted cumene; (f) a step of recovering styrene from at least a portion of the fraction including styrene of step (e) and distilling and separating the fraction including unreacted ethylbenzene; (g) a step of recovering ethylbenzene from at least a portion of the fraction including unreacted ethylbenzene of step (f) and recycling to a reaction part, and distilling and separating the fraction including benzene and toluene; (h) a step of recovering cumene from at least a portion of the fraction including unreacted cumene of step (e) and recycling the recovered cumene to a reaction part, and distilling and separating a fraction including alpha-methylstyrene; and (i) a step of recovering alpha-methylstyrene from at least a portion of the fraction including alpha-methylstyrene of step (h), and distilling and separating a fraction including a remaining component having a high boiling point,