The generation of high degree of quantum squeezing based on the interactions between two optical fields and coherent population trapping (CPT) medium has been investigated. With the inherent nonlinearity arisen from atom-light interaction and the slow light propagation in CPT, the quantum noises of the two input fields which are coherent states can be greatly reduced below that of vacuum with a high optical density (OD) even in a single passage. Our study reveals that the noise squeezing of more than 10 dB can be achieved with an OD of 1,000, which is currently available in experiments. Besides, the input light intensity and two-photon detuning are key factors in this process; we also demonstrate that the minimum quadrature variance at a given OD can be reached for a wide range of these two factors, showing the proposed scheme is flexible and robust. Our introduction of high OD in atomic media not only bridges a long light-matter interaction time comparable to optical cavities, but also open new avenue in the generation of squeezed light for quantum interface.