Nowadays, the array signal processing is becoming more and more popular. There are a lot of applications based on the array signal processing. One of the key application is the phase array radar. Since phase array radar has a huge amount of antenna elements, we need to perform subarray level processing. In this paper, we propose the subspace projection adaptive approach to calculate the adaptive weighs at the subarray level for digital adaptive beamforming. The proposed method helps to decrease the hardware cost and make the phase array radar available to suppress the interference adaptively. During the calculation of the adaptive weights, the inverse of the matrix needs to be derived. The float round-off error in the calculation is accumulated. It is very important to analyse the impact of the float round-off error on the result accuracy. In this paper, the float round-off error in the proposed method is analysed.