A solid state disk, a data transmitting method and an intermediary controller thereof are provided. The solid state disk includes at least two flash memories, a SSD controller and an intermediary controller. The intermediary controller is connected between the flash memories and the SSD controller. The intermediary controller includes at least two flash interfaces, a customized interface and a data buffering unit. The flash interfaces are connected to the flash memories. The customized interface is connected to the SSD controller. The intermediary controller has a first clock domain and a second clock domain. The first clock domain is used for transmitting data from the flash memories to the data buffering unit. The second clock domain is used for transmitting data from the data buffering unit to the SSD controller. A frequency of the second clock domain is higher than a frequency of the first clock domain.