A method and an apparatus for transmitting a PPDU in a WLAN system are proposed. Specifically, a transmitter generates a PPDU and transmits the PPDU to a receiver. The PPDU includes a legacy field and an EHT field. The legacy field includes a VHT-STF and a HE-STF. The EHT field includes an EHT-STF and a data field. The EHT field is configured on the basis of a CSD value of each transport chain. The CSD value is determined as a candidate CSD value in which the sum of a first absolute value and a second absolute value is the minimum on the basis of a power ratio. The power ratio is a ratio of a received power of the VHT-STF, the HE-STF, or the EHT-STF to a received power of the data field. The first absolute value is an absolute value of a value related to 5 percent of a CDF relative to the power ratio. The second absolute value is an absolute value of a value related to 95 percent of the CDF relative to the power ratio.