Based on microstructure of as-cast Ti-6Al-4V titanium alloy, an image processing to simplify model of β grain size, α lamella space was developed and a quantitative statistical method of β grain size, α lamella space in two-phase titanium alloys was established, with which β grain size and α lamella space of as-cast Ti-6Al-4V titanium alloy with different plate thickness were analyzed. The relationship between β grain size, α lamellar space and cast plate thickness was discussed. The results show that β grain size and α lamellae thickness nearly linearly increase as plate thickness increasing. β grain size has normal distribution and with cast plate thickness increasing, the dispersion gets larger.