A forward stack decoding (FSD) algorithm for rateless spinal codes is proposed in this letter. This decoding algorithm divides the decoding tree of the spinal code into several layers, and then searches the decoding paths in each single layer. Compared with the bubble decoder, which was initially designed for spinal codes, the proposed FSD algorithm can decrease the complexity significantly without sacrificing the rate performance. We also illustrate that the complexity of the FSD algorithm decreases with the increase of the signal-to-noise ratio (SNR). Simulation shows that the complexity of the FSD algorithm is only 15.37% of that of the bubble decoder when SNR is 20 dB.