Optical spectrum analysis has been widely used in numerous areas such as optical network performance monitoring, materials analysis and medical research. Although there are many kinds of spectrometers, on-chip spectrometer could be a promising alternative with apparent size and weight advantages,. Silicon-on-Insulator(SOI) waveguide technology offers means to miniaturize the different parts of the spectrometer, even if often at the cost of performance and scalability. In this work, a cascaded waveguide structure was proposed for a spectrometer, with a spectral range from 1150nm to 1550nm, which corresponds to the second overtone region of the NIR absorption, and a resolution of 2 nm for performing spectrum derivation. The spectrometer is realized by a SOI cascaded Mach-Zehnder Interferometer and four SOI arrayed waveguide gratings. The cascaded MZI based coarse wavelength division de-multiplexers was employed for the first stage of the spectrometer and was used to disperse the signal into four channels. The output signals of the four channels are further dispersed into eight channels by the second stage AWG structures. We further implemented the thermo-optic modulation to achieve a higher spectral resolution. The output channel wavelengths of the spectrometer are modulated (with a wavelength shift 2 nm) by the embedded heater to obtain the first order derivative spectra of the input optical signal. We present the theory, modeling, and experimental demonstration of the thermally tuned spectrometer. With respect to the computer simulation and device characterization results, the 400nm spectral range and the 2nm resolution have been demonstrated.