The disclosure claims a transmission method and device in WLAN, wherein the method includes: a transmitting STA or a receiving STA performing channel detection and/or channel scanning on a channel of the first channel set and/or the second channel set; if the result of the detection and/or scanning satisfies a preset condition, the transmitting STA and the receiving STA exchanging a reservation message on the channel of the first channel set or the second channel set, wherein the reservation message is configured to reserve a channel of the second channel set for transmission; the transmitting STA transmits a data frame to the receiving STA on the reserved channel of the second channel set; and after receiving the data frame, the receiving STA transmits acknowledgement information of the data frame on the channel of the first channel set or the second channel set.