Abstract: Netted radar systems show great potential in improving the performance of radar detection, tracking and interference suppression. However, the systems suffer high auto-correlation and cross-correlations of transmitted waveforms. Meanwhile, they also have to face the congested spectrum environment, especially when some radars in the net working on High Frequency (HF) to Ultra High Frequency (UHF) band. To solve this issue, a new method for designing sparse frequency unimodular waveform with low range side lobes is proposed, which minimizes a new effective penalty function based on both requirements for the Power Spectrum Density (PSD) and Integrated Sidelobe Level (ISL). An iterative algorithm based on FFT and subspace decomposition is proposed. The numerical examples show that the proposed approach is efficient in computation and flexible in designing sparse frequency waveform with low auto-correlation and cross-correlations.