With the booming development of low earth orbit (LEO) satellite constellations, enhancing the global navigation satellite system (GNSS) performance based on LEO satellites is attracting more and more research attention. Compared with medium earth orbit (MEO) satellites, LEO satellites have faster motion speeds and more rapid geometric changes. When combined with GNSS, they can effectively shorten the required convergence time of precise point positioning (PPP), which is one of the main advantages of LEO navigation augmentation system. To achieve this goal, a dedicated LEO navigation augmentation signal needs to be broadcasted, and the signal needs to meet the following design requirements: (1) It can support high-precision carrier phase measurement to achieve PPP function (2) Can provide high data rate to meet the broadcasting needs of precise navigation messages (3). The signal frequency band should be within the GNSS frequency band to reduce the complexity and cost of GNSS/LEO receivers (4) the spectrum of LEO navigation augmentation signal should be separated from the GNSS signal spectrum as much as possible to reduce interference with the GNSS signal. This paper takes the GNSS L1 and L5 frequency bands as examples to conduct a preliminary design of LEO navigation augmentation signal. From the perspective of reducing interference to GNSS signals, the carrier frequency of the LEO navigation augmentation signal is selected, and the modulation type is designed. In order to meet the requirements of both high-precision measurement and high data rate broadcasting, it is proposed that the LEO navigation signal consists of a measurement component for modulating low-speed messages and a data component for modulating high-speed messages. These two signal components are combined into one signal using multiplexing code shift keying (MCSK) method. On this basis, compatibility assessment was conducted, and the impact of LEO navigation signals on GNSS signal was further analyzed.