A heater for heating and vaporizing droplets in gas stream, comprising a heater housing and a heater body located inside the heater housing, wherein the heater housing is provided with an airflow inlet and an airflow outlet, the airflow enters into the heater housing via the airflow inlet, flows through the heater body, and then is discharged via the airflow outlet; the heater body comprises a stereoscopic network structure formed by interweaving one or more electrical heating wires. The use of the heater and a method for preparing isocyanate using the heater. The heater has a simple structure, a low pressure loss, uniform heating and a high heat utilization ratio during preparing isocyanate.