Unsupervised Post-Training for Multi-Modal LLM Reasoning via GRPO
Authors: name: Lai Wei; name: Yuting Li; name: Chen Wang; name: Yue Wang; name: Linghe Kong; name: Weiran Huang; name: Lichao Sun
Published: 2025-05-28 15:11:16

Abstract: Improving Multi-modal Large Language Models (MLLMs) in the post-training
stage typically relies on supervised fine-tuning (SFT) or reinforcement
learning (RL). However, these supervised methods require expensive and manually
annotated multi-modal data--an ultimately unsustainable resource. While recent
efforts have explored unsupervised post-training, their methods are complex and
difficult to iterate. In this work, we are the first to investigate the use of
GRPO, a stable and scalable online RL algorithm, for enabling continual
self-improvement without any external supervision. We propose MM-UPT, a simple
yet effective framework for unsupervised post-training of MLLMs. MM-UPT builds
upon GRPO, replacing traditional reward signals with a self-rewarding mechanism
based on majority voting over multiple sampled responses. Our experiments
demonstrate that MM-UPT significantly improves the reasoning ability of
Qwen2.5-VL-7B (e.g., 66.3 %rightarrow72.9 % on MathVista, 62.9
%rightarrow68.7 % on We-Math), using standard dataset without ground truth
labels. MM-UPT also outperforms prior unsupervised baselines and even
approaches the results of supervised GRPO. Furthermore, we show that
incorporating synthetic questions, generated solely by MLLM itself, can boost
performance as well, highlighting a promising approach for scalable
self-improvement. Overall, MM-UPT offers a new paradigm for continual,
autonomous enhancement of MLLMs in the absence of external supervision. Our
code is available at https://github.com/waltonfuture/MM-UPT.