The invention discloses an online monitoring method for a VOCs (volatile organic compounds) fixed source. The online monitoring method includes: under the normal working conditions, performing full-scanning analysis on gas emitted from the VOCs fixed source to obtain the ionic strength range of each characteristic ion; calculating to obtain the total equivalent VOCs concentration emitted from theVOCs fixed source under the normal working conditions; performing full-scanning analysis on to-be-monitored gas emitted from the VOCs fixed source in real time by adopting an online mass spectrometerand calculating to obtain the total equivalent VOCs concentration of the to-be-monitored gas; comparing a full-scanning mass spectrometry of the to-be-monitored gas and the total equivalent VOCs concentration with a full-scanning mass spectrometry of the gas emitted from the VOCs fixed source and the total equivalent VOCs concentration under the normal conditions and judging whether VOCs fixed source emission is abnormal or not at the moment. With the online monitoring method, monitoring of a single VOC index and total index of non-methane hydrocarbon is not needed, and restriction of a current VOCs monitoring method standard system is effectively avoided.