The netted radar often suffers congested spectrum assignment, high autocorrelation sidelobes as well as cross-interference of transmitted waveforms. In this paper, the Joint Optimization Relaxed Alternating Projection (JORAP) method is introduced to design sparse frequency waveforms with low auto- and cross-correlation sidelobes under the unimodular constraint. Firstly, the original optimization issue is converted into spectrum approximation issue via FFT between the aperiodic correlation function and power spectral density. Secondly, different design requirements are synthesized via multi-objective optimization mechanism. Next, the projection space is exploited utilizing its relaxed factor and accelerating factor. Finally, the iterative optimization is conducted by FFT and accelerated alternating projection. Simulations demonstrate that this algorithm, without computing conjugate gradient, can avoid the local stagnation and obtain efficient performance, which seems more convenient for engineering than some prevalent alternating projections or cyclic algorithms.