Accurate and efficient prerouting timing estimation is particularly crucial during placement to alleviate time-consuming design iterations. Machine-learning (ML)-based methods have been introduced recently to predict the post-routing timing results at placement stage, but most of them neglect the impact of timing optimization during physical design, suffering from accuracy loss due to inconsistent circuit netlist. In this work, an optimization-aware prerouting timing prediction framework based on multimodal learning is proposed to calibrate the timing changes between placement and routing stages, where the local netlist and layout information are extracted by graph neural network (GNN) and convolutional neural network (CNN), respectively, while the global information along the path is further extracted by Transformer network. Based on the predicted post-routing timing results by the proposed framework, timing optimization guidance is generated to enhance traditional design flow with better physical implementation quality. Experimental results demonstrate that for the OpenCores benchmark circuits under TSMC 22nm process, the proposed framework achieves significant correlation and accuracy improvement with an average of 0.9219 in terms of R2 score and 2.12% of mean absolute percentage error (MAPE) as well as an average runtime acceleration of$645\times $compared with traditional design flow on testing designs. With the timing optimization guidance, significant worst negative slack (WNS) and total negative slack (TNS) improvement are achieved compared with traditional flow after placement and routing, respectively, without noticeable area, power, wire length, and the number of design rule check (DRC) violations increase.