Since real-world legal experiments are often costly or infeasible, simulating legal societies with Artificial Intelligence (AI) systems provides an effective alternative for testing and advancing legal theory, as well as supporting legal administration. Large Language Models (LLMs), with their world knowledge and role-playing capabilities, are strong candidates to serve as the foundation for legal society simulation. However, the application of LLMs to simulate legal systems remains underexplored. In this work, we introduce **Law in Silico**, a unified LLM-based agent framework for simulating legal scenarios that incorporate individual decision-making and institutional mechanisms, such as legislation, adjudication, and enforcement. We calibrate agent behaviors against real-world crime data, demonstrating that LLM-based agents can capture realistic sociological correlations. Building on this foundation, we structure our simulation through a ”Micro-to-Macro” process: we conduct micro-level simulations in representative conflict-driven scenarios, allowing legal rules to evolve through agent-institution interactions naturally. These evolved laws are then deployed back into macro-scale populations to evaluate their effectiveness in regulating behaviors. Through comprehensive experiments, our results reveal that a well-functioning, transparent, and adaptive legal system can mitigate "cat-and-mouse" regulatory dynamics and offer better protection for vulnerable individuals.