A method and apparatus for checking a data frame length relate to an Ethernet passive optical network in the communication field. The method includes: during reception of an Ethernet frame, when determining through analysis that a received Ethernet frame is a data frame, extracting frame length information and frame indication information in the data frame, and calculating a frame length mantissa according to the frame length information; writing the data frame into a small cache for storage according to the frame indication information, and counting write enable signals used for controlling writing of a data frame by using a base-N counter, so as to obtain a count value of the write enable signal; and comparing the frame length mantissa with the count value, and if a comparison result is consistent, then a data frame length being successfully checked, and writing the frame length information into the small cache for storage.