Online portfolio selection has been actively studied to maximise overall returns by selecting the optimal portfolio weights using online algorithms. However, most work has focused on long-only portfolios, and developing efficient algorithms with loose portfolio constraints remains a challenge. In this letter, the classical online portfolio selection problem is reformulated to allow long/short and margin. For this problem, conventional gradient-based online algorithms face the challenges of high regret and computational complexity due to non-optimal gradients and high-dimensional projections. To tackle this, we propose a novel online algorithm that introduces mirror descent to achieve dimension-free regret in a non-Euclidean space. Specifically, a Bregman divergence is introduced to replace the $\ell _{2}$ norm as a valid proximal setup for the problem to achieve uniform gradients and reduce projection computations. Furthermore, a smoothing technique is developed to reduce the variance of the gradients. The evaluation shows that our algorithm achieves low regret bound and computational complexity, which guarantees a 30% advantage over other strategies in Chinese futures market.