We demonstrate a proposal for making an ultrastable laser referenced to a multi-cavity, enabling a lower thermal noise limit due to the averaging effect. In comparison with a single-cavity system, relative frequency instability of the synthesized laser can be improved by a factor of the square root of the cavity number. We perform an experiment to simulate a two-cavity system with two independent ultrastable lasers. Experimental results show that the relative frequency instability (Allan deviation) of the synthesized laser is 5 × 10 ¹⁶, improved by a factor of √2 from a single-cavity-stabilized laser.