The transportable optical clock can be widely used in clock comparison, timekeeping, precision measurement, dark matter detection, and so on. The development of transportable optical clocks is a significant direction of optical clocks. It is an important piece of equipment for the comparison of different types of optical clocks and the measurement of gravitational redshift. It is the crucial step to develop a transportable cooling light source for the preparationof cold atoms. This letter mainly introduces the development and application of the transportable light source for the second-stage cooling. Firstly, we simulated and analyzed a transportable cubic ultra-stable cavity with a length of 25 mm and the fineness is measured to be 1.94×105. Then we locked a 689 nm semiconductor laser on the cubic ultra-stable cavity through the Pound-Drever-Hall frequency stabilization technology to obtain a narrow linewidth frequency stabilized laser source at the wavelength of 689 nm. After beating with a reference laser, the linewidth of the frequency stabilized laser is measured to be less than 263 Hz, and the frequency stability is better than 1.56×10-14 at one second, showing the laser source can be used for the second-stage cooling in a strontium optical clock. In addition, to verify the application of the laser source, two slave lasers with the same performance were prepared by injection locking technique, which was used as cooling and stirring light respectively in the second-stage cooling. All optical parts are integrated on a 0.56m2 optical breadboard to achieve transportability and fiber-coupled on the vacuum system so as to be easily moved. At last, we applied the transportable system tothe preparation of a cold strontium cloud. As a result, 2×106 atoms with a temperature of 5.3K are prepared in the second-stage cooling experimentally, which laid the foundation for optical lattice loading and clock transition spectrum detection of cold atoms in the transportable strontium optical clock.