A method and a device for transmitting a PPDU in a WLAN system are proposed. Specifically, a transmission device generates a PPDU and transmits the PPDU to a reception device through a 320 MHz band. The PPDU includes a legacy preamble and an EHT field, and the legacy preamble includes an L-STF and an L-LTF. The legacy preamble is generated by applying a first phase rotation value or a second phase rotation value. The first phase rotation value is obtained based on a third phase rotation value and a fourth phase rotation value. The third phase rotation value is a phase rotation value obtained by repeating a phase rotation value defined when the PPDU is transmitted in an 80 MHz band four times. The fourth phase rotation value is a phase rotation value defined for each 80 MHz band in the 320 MHz band based on an optimal PAPR of the L-STF.